Where do all the bullet casings after a war end up?

The majority of tonnage of brass shell casings ended up in recycle. Anecdotally; it was such a substantial amount that a friends grandfather had a contract with the US Navy to collect brass shell casings in the Pacific theater ( no details).

In Gaza they use spent shells as flower pots.

Hi I believe I can shed a little light on some of the uses for the shell casings after the war. About 2004 or thereabouts my sister, Frankie Heffernan and I lived in Raymond, WA and I was running a small shop that carried a variety of things including antiques and collectibles and I always had my sister looking for things for my shop.

In late 2004 she called me to tell me about some friends she had in Gig Harbor that were moving to Montana to be closer to their kids and they had a lot of things that they weren't taking with them so they were going to give me first choice if I came right away. Sis and I went up that day with a large moving truck because I had no idea what we were going to buy.

This was an elderly couple who were very gracious and had a home that he built by himself and it was not only breathtaking to see but it was also absolutely spotless and this was a 2 story home. In the living room were three large oriental cabinets with another one upstairs in their bedroom..I hope I still have a picture of one of them so you can see how beautiful they were. Each cabinet which measured 48 in length and 24 deep by 36 tall, had over 100 pieces of ornate brass including 8 pieces for the 4 way locking system that each one had.

I fell in love and bought the 3 that he was going to let go of for a very reasonable price and that when he told me about the cases. He was a pilot in WWII and stationed in Okinawa and there was also a Korean man who lived close to him and he built beautiful wood cabinets of all sizes. He became friends with the craftsman and asked if he could build him 4 large cases that would all have the kind of locking mechanisms that he put on others.

The cases were started in 1949 and then shipped to the U.S. in late 1950 and the pilot got to watch how the cases were made and that's when he found out that the brass pieces were from the shell casings that were found by a couple of his friends and sold to the Korean craftsman to melt down and prepare for use on the cases. The pilot said it was time consuming but the end product was unbelievable. I once had all of the information written down but I have moved several times and everything connected to my cases was lost. I will now make a better attempt at finding them.

Shells, tanks, etc., left behind in North Africa in WWII were so numerous that scrap metal was Libya's primary export for most of the 1950s (Ellen Lust, The Middle East, p.623). (Significant oil reserves weren't discovered there until 1959.)

There are tons of unexploded shells ploughed up every year in the lowlands of Belgium and Northern France. Its known as the iron harvest.

While driving round the countryside of former battlefields you will see what looks like log stores at the gateway to fields or at crossroads but when you look closer you realise that the logs are unexploded ordinance. The army collects from these local collection points periodically and destroys them by controlled explosion in deserted areas. Some of them are armour piercing, some high explosive but by far the most sinister are the gas shells left over from both major conflicts.

As Michael says, people collect them as souvenirs. Military and other government agencies also collect them to prevent accidents.
Even to this day farmers in Europe plow up munitions and other trash from WW1 and WW2 regularly, mines, torpedoes and aircraft bombs are dredged up in fishing nets and when dredging rivers and harbours to deepen and widen shipping channels.
There's still areas (mostly forests but also coastal dunes) all over Europe that have warning signs around them warning against entry because they're WW2 minefields that have never been cleared.

Removing all the shell casings and bullet casings and melting them down for the metals may seem like a profitable idea, but if you consider that they're spread rather thin over a very large area, the work involved is extremely expensive. And what with the land being owned by many people, just getting permission from all those owners is more than tedious.
And of course after decades most of it will be buried, sometimes meters deep.

Well, people from the generation that grew up right after the war told their kids (including me) of the treasure trove of unexploded ammunition they came across as the kids. The stories included numerous kids loosing finger or getting burned by powder, as well as occasional fatal explosions. Some schools even invited explosives experts to tell the kids how to handle them safely, based on the assumption that boys will be boys and won't leave ammunition alone, and the knowledge of how to handle it safely may save their lives.

The dangers of unexploded ammo didn't end with that generation though. A elementary school classmate of mine brought to school a mine he found somewhere so that he and his friends could take it apart and see what's inside. The three second graders lost their lives that day.

So-called "black archaeology" persists to this day in parts of Eastern Europe: people are digging up 70-year-old battlefields to find something valuable for sale, usually helmets and daggers, sometimes guns.

As for recycling casings, they were usually made of brass, a valuable metal, and there were plenty of location where one could sell them by weight.