Hey @celestia!
If they’re assets from the marketplace, you use the launcher on the “Library” tab under “Vault” to add things to a project. If you’re migrating, it goes from one project to the other, no middle stop. You can pack things to add that middle stop using right click → “Export” on the original and then “Import” on the target project under “File” (I think, if not file, another top bar tab on the main page).
As far as adding .uassets, I believe the reason you can’t simply drag and drop .uasset files into UE is that every file in UE5 has an ID#, and when a file is converted to .uasset it gains that ID#. If a file had the same ID# as another .uasset, when that file is sought out it would return two answers for one question and that would be a crash-worthy error, and that ID# cannot be changed due to integral code in the engine.
That’s the reason when you get things from the store or elsewhere, there is a process to that, you can’t just drop files in. They need to be packages, like .fbx or .obj.
I really hope that helps. I’m sorry to be the bearer of bad news.