How did the Soviets decide what to annex in WW2?

This answer is according to the original version of the question, which read: "Is something known about the Soviet post-occupation plan for Finland?"

Regarding Post WWII Finland - truth is, I thought the question was weak on this point, because it gave no reason or substantiation regarding possible "Soviet post-occupation plan for Finland". But I did find this:

Why didn't USSR occupied Finland in 1944?

Why didn't USSR occupy all Finland in 1944... but the super-power that USSR was by 1944, could have occupied the country...so why not?

From a Finn, apparently quite knowledgable on this subject - see the page indicated for extensive details: Here's something I wrote, years ago, in answer for this question

The short answer: Stalin didn't want a revolution in Finland unless the Finnish communists could effect one themselves, without Soviet help. And the Finnish communists were unable to make a revolution without Soviet tanks rolling into Helsinki....

These are the most important reasons why the Finnish communists were unable to make revolution without the Soviet help. But why didn't Stalin give that help? Why didn't the Red Army occupy Finland and put the communists in power?

1) As the Soviets very well knew, the Finnish Army remained an effective fighting force. After the Soviet offensive on 9 June 1944, the following two weeks were certainly not the most glorious chapter in the history of Finnish Army. But what was most important is that the Finnish Army retreated in orderly fashion and remained intact and undefeated in the field. In the fierce battles of late June and early July 1944 the Red Army was fought to standstill, and despite its efforts, Red Army was unable to occupy Finland. As late as early August 1944 two Soviet divisions were encircled and destroyed in northern Karelia near Ilomantsi. Fully mobilized, Finnish Defence Forces fielded 450 000 experienced men. As Stalin himself in 1948 said to a surprised Finnish delegation: "Nobody respects a country with a weak army. Everybody respects a country with a strong army. I propose a toast to the Finnish Army!"

Occupying Finland would have meant for the USSR bloody war right after the devastations of WWII at the time Cold War was beginning. In all probability it would have been similar experience like Chechenia is for Russia today. When the Continuation War ended in September 1944, a group of Finnish general staff officers (with Mannerheim's unspoken approval - that's plausible deniability 40 years before Iran-Contra!) began secretly to organise weapon caches around Finland. They were meant to be used to support large-scale guerilla warfare if USSR tried to occupy Finland. This so-called Weapon Caches Case became soon public and offical investigations began (conducted, of course, by the communist Security Police). For the Soviets it was yet another evidence that if they tried to occupy Finland, they had to pay dearly. Decades later, Molotov told to a party historian: "It was a very wise decision [not to occupy Finland]. It would have been a bleeding wound in our side! The people there, they are very stubborn, very stubborn."

From these points it clearly emerges that a non-communist Finland was in best interests of post-war USSR. If the Finnish communists were able to take power themselves, good, but as they were manifestly unable to do so, better leave Finland in peace. An attempt to occupy Finland would only mean engaging USSR so soon after the WWII in a messy and costly conflict that would damage its economy and foreign relations.

From the preparation of the Finnish general staff officers for a large-scale guerrilla war if USSR tried to occupy Finland, and from Molotov's remarks, it seems clear that there was fear in Finland, and talk in the USSR, about an invasion of Finland, in the near-end and immediate post-war period, because there was already a significant communist presence there at that time. But the USSR opted not move into Finland, because they knew it would have been much too messy.

With regards to Finland, it was the second country that the USSR invaded after signing the Molotov-Ribbentrop Pact, where Finland was one of the countries that had been assigned to the Soviet "Sphere of influence" together with Estonia, Latvia, Bessarabia and half of Poland. (The first country invaded was of course Poland).

However, there is a dispute amongst historians of what the aim of the war with Finland was, and there seems to be no papers in the Soviet archives providing a definite answer.

Officially the cause of conflict between Finland and USSR was a dispute over Karelia, which the Soviet Union wanted to provide a buffer for Leningrad. The USSR shelled one of their own border posts and blamed it on Finland as an excuse to invade. Stalin seem to have thought that Finland was going to be a soft target and that they would be able to just walk in and take over. Just as in the cases with the Baltic States the Soviet Union declared a communist republic with a puppet government. In the cases of the Baltic States these puppet governments were quickly included in the USSR, but as Finland never surrendered this wasn't an option with the short-lived Finnish Democratic Republic.

So we do know that the USSR intended to create a puppet state, because they actually did so. The remaining question is if they intended to annex that puppet state into the the USSR or let it stay a nominally independent state, like Poland and East Germany.

The arguments for annexation into the USSR are:

1. All the other regions invaded during 1939-1940 was annexed into the USSR in one way or another.
2. All the regions invaded during 1939-1940 had earlier been Russian, and this includes Finland. This is probably one primary reason for the annexation of these regions; the Soviet leadership simply viewed these territories as rightly belonging to them.
3. Molotov during a visit in Berlin in 1940 was asked by Hitler how they planned to settle "The Finnish question" and answered that they planned to do it the same way as they had with the Baltic states.

Arguments against annexation are:

1. Finland was even under Russian rule semi-independent.
2. The Soviet Union said that they were not going to make Finland a Soviet Union.
3. The Soviet Union didn't annex Finland, even though they could have.

I find argument 2 naive to the extreme, and I think argument 3 somewhat strange. Of course the Soviet Union technically could have occupied all of Finland, Finland was a small country with few people, the Soviet Union a huge empire. But the Finnish resistance was much stronger than anticipated, and occupying Finland would have cost the USSR a lot of men, that was better used elsewhere. It's clear that the USSR decided that the cost of Finland was not worth it, so they instead made peace. I don't think that tells us anything about what their plans for Finland was, had they succeeded in occupying it.

As such I think the weight of evidence falls clearly to the side that the USSR intended to annex Finland in the form of a USSR state, although it's unlikely we'll ever know for sure.