@JMS's answer is excellent for commodities such as food. I'd like to add a few points that caused civilian hardship even without blockade.

Labor shortages

World War I saw for the first time millions of men and women mobilized and sent to war. This means millions of people no longer doing their regular jobs. Instead they now need to be fed, clothed, and supplied by the shrunken workforce. Worse, as soldiers they need more than as civilians, and battles consume men and material at an alarming rate.

To avoid losing too many specialized workers, many people who have critical jobs are exempted from serving: farmers, factory workers, etc. In addition the loss of workers compensated for by transferring more people to manufacturing, and also opening up manufacturing to new groups such as women. It's no coincidence that Women's suffrage got a big boost after World War I.

Nitrogen

Nitrogen is critical for two things: explosives and fertilizer. The Great War required a lot of explosives, and unless the production of nitrogen picked up the availability of fertilizer would suffer.

Even though the atmosphere is mostly nitrogen, for a long time we didn't know how to use it. Instead we mined it. The Allies had access to natural sodium nitrate and bird poop, but the Germans were cut off.

Instead they turned to the then-new Haber process to fix nitrogen from the air and produce ammonia which could then be turned into fertilizer and explosives. But this was a brand new process and production was still ramping up when the war started.

Critical war materials

There are some materials which are simply not locally available and there are no replacements.

Petroleum, while not nearly as important in WW1 as in WW2, was still necessary for lubricants and fuel hungry naval vessels. Germany had some supply from Romania until they joined the Allies in Aug 1916.

Rubber, again not as important in WW1 as in WW2, was also necessary for gas masks, elastic, and seals. At the time nearly all rubber was imported natural rubber from trees in South America, South and Southeast Asia. Synthetic rubber wouldn't become economically viable until WW2.

Others are not so obvious. WW1 required a lot of steel. Steel requires a lot of coal and iron, of which Germany had plenty. But good alloy steel requires alloying elements like manganese, nickel, chromium, molybdenum, vanadium, silicon, boron, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium. A shortage in any one of these materials can cause good steel to become scarce.

Others are necessary to keep people happy: coffee and tobacco, for example.

Horses

Horses were also needed by the military in great numbers, plus the specialists to handle them, shelter, and equipment. Horses were not just for cavalry, but were much more critical for transportation: trucks being rare and roads being poor. And they died in alarming numbers. This took them away from civilian use, much of the economy was still horse-drawn in World War I. In particular farm production was greatly impacted.

And these great numbers of horses needed to be cared for and fed. Horse fodder became a critical commodity.

The Great War has a good piece on Animals of World War 1.

Rolling stock

In WWI if you wanted to move and supply an army of millions there were two ways to do it: horses or trains. We talked about horses already. Moving and supplying armies requires a lot of rolling stock: locomotives, cargo cars, passenger cars, etc.

Trains are also how you move food, goods, and civilians around. A country has a limited amount of rolling stock. The more the military uses, the less the civilian economy has available. This hurts production.

Germany had no plan

Germany, in particular, produced most of its own food and thought its continental supply lines would keep trade flowing. But they didn't have a wartime production plan. Germany didn't plan on a war, it was a local incident that spiraled out of control. And they didn't plan on a long war, they thought they could knock France out in a few weeks and turn all their attention on the Russians. And they did not plan on Britain and the Royal Navy getting involved, the Germans did not believe Britain would actually go to war to protect Belgium.

Without a plan, food production was hit hard by the loss of men and horses and fertilizer. They didn't expect to have to keep an army of millions in the field for years. Nobody did.

Britain was not capable of feeding itself even back then but it sat at the head of an Empire. 60% of its food supply was imported, mostly from the colonies. UK farming is still subsidised in part as a strategic concern to ensure that a local food supply can be maintained to offset any temporary disruption to supply.

Germany at the time was importing a third of its food supply so a lot more independent than the UK. During the war though, the lack of manpower available for farming meant reduced work done which women couldn't pick up the slack for. At the same time animals starved as a third of fodder was imported too. By the end of the war agricultural output had halved. Germany like the UK used pows to work its farms but they were not enough to offset all the men gone to war.

In an age where horses were important to farming, the loss of horses to the military also caused problems with production. As an example, the USA sent a million horses to help the allies. The American army took almost 200,000 more for its own needs in Europe.

American farms largely offset the loss in trade to the allies by increasing production. After the war they found themselves overproducing foods and farms couldn't sell their product. It was one of the factors that lead to the Great Depression.

I interpret your questions as all dealing with events and capabilities prior to WWI and the havoc that played on the European economies.

Question:
Weren’t Germany and Britain self maintaining countries and economies already before World War I?

Germany and Britain both had modern economies prior to WWI. That means both relied on exports and imports to feed their economies and fuel their industry as efficiently as possible.

Question:
What were they doing before war to provide for its people?

They were industrializing.

Germany like United States began it's industrialization late. (1870's) Germany began after its unification, the United States after the American Civil War.

The Economy of Germany
The rapid advance to industrial maturity (between 1870-1914) led to a drastic shift in German economic situation, from a rural economy into a major exporter of finished goods. The ratio of finished product to total export jumped from 38% in 1872 to 63% in 1912. By 1913, Germany came to dominate all the European markets. By 1914, Germany became one of the biggest exporters in the world.

Britain was the first to industrialize in the 1700's.

Economic history of the United Kingdom: 1700 to 1900
The 18th century was prosperous as entrepreneurs extended the range of their business around the globe. By the 1720s Britain was one of the most prosperous countries in the world, and Daniel Defoe boasted:

we are the most "diligent nation in the world. Vast trade, rich manufactures, mighty wealth, universal correspondence, and happy success have been constant companions of England, and given us the title of an industrious people."

While the other major powers were primarily motivated toward territorial gains, and protection of their dynasties (such as the Habsburg and Bourbon dynasties, and Prussia's House of Hohenzollern), Britain had a different set of primary interests. Its main diplomatic goal (besides protecting the homeland from invasion) was building a worldwide trading network for its merchants, manufacturers, shippers and financiers. This required a hegemonic Royal Navy so powerful that no rival could sweep its ships from the world's trading routes, or invade the British Isles.

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Question:
Where they so reliant on other countries to basically survive?

Reliance on international markets is a byproduct of Industrialization. In 1776 Adam Smith wrote "Wealth of Nations". In this famous book chapter 1, Smith describes business examples where by using "Division of Labor" the skill level of workers could dramatically decrease while the net production dramatically increase. This meant instead of one master craftsmen providing one good for a street or town, many less skilled workers, working together, could provide more than enough of a specific product for the entire country. Productivity grew exponentially as specific jobs in a products manufacturing chain were specialized. Smith's famous example was the manufacturing of pins. Making pins in Smith's day could be broken down into 18 distinct steps. Straitening the wire, pointing the wire, cutting the wire, even folding the paper and packaging the pins were isolated steps. Where one master craftsmen could produce 20 pins a day performing each step himself. 10 men who were not master craftsmen but simplely taught one step of the process could produce 4,800 pins a day.

Adam Smith's Pin Factory
The increase in labor productivity (output per person) is as high as 50 times that of individual pin makers.

The reduction in unit cost or average cost (AC) and the huge increase in quantity produced do not just replace older methods of organization and production. They increase the potential “extent of the market.” (Chapter 3 – “That the Division of Labour is limited by the Extent of the Market”) Existing users not only buy more pins at the lower price but also think up new ways to use cheaper pins. The geographical limits of the pin market expand; contemporary and future reductions in transportation costs further expand domestic markets and increase exports.

Using industrial techniques such as division of labor and latter automation and powered tools, industrialized countries like Britain and Germany were able to vastly increase productivity in specific sectors and improve their overall economic efficiency. A necessary side effect however was a reliance on international trade to sell excess products. A byproduct of exports is imports. The same infastructure which which enables exports also encourages, supports and subsidizes imports:

• freedom of the seas
• relationships with foreign countries
• large merchant fleet

Products which are not industrialized could be imported by returning ships after delivering exports. That's not a bad side effect either. It's an efficiency, one of many which came through industrialization. Broader markets, replacing more expensive products with less expensive alternatives.

Sources:

• The Economy of Germany
• Economic history of the United Kingdom: 1700 to 1900
• Wealth of Nations
• Division of Labor
• Adam Smith's Pin Factory