Arhive folder ?

No, they have not moved there automatically, like on a schedule. That said, if you move an item from one vault to another, you may find that the copy that was in the original vault is now in the archive. That's because the move action is a copy-and-then-delete type event for that original item. The new (moved) copy would reside in where you moved it to. This process is due to each vault being independent.

It's possible that some of the things were there in what was formerly the trash before the change took place. I personally do not clean out the items there and leave them as is. It's like a storage bin for me. I've sent so much stuff there that it's an out-of-sight, outta-mind sort of thing now. I need to take action and clean up some items, but that's another story for another day. They can remain there if you like, or you can spring clean if you wish. If you start spring cleaning, ensure the copy in the archive is elsewhere on your system. It is possible it exists now by a new name and so on. Only you can make this determination. If so, you should be good to remove it if needed.

One suggestion is to take it slow and methodically, speaking from personal experience. It's really easy to think you are working in the archive and find yourself in the vault itself. The story's moral is to make sure what's cleaned up should be. We would not want you to remove any data you need to keep.

My primary use is to think of it as a very high shelf. When I am working with my data, instead of deleting a login, for example, I put it on the high shelf so no one (cough me) will accidentally delete it until absolutely and explicitly told to do so. Just in case I need it again. Yes, I fully protect things from my spring-cleaning nature. Haha
I just finished cleanup of all of my vaults. Now is a perfect time to start on the archive. Thanks for the nudge; looks like my weekend has a goal now.