There is no reason to feel stupid. Browser containers are tricky and relatively new, and I was bitten by some quirks, too. Nevertheless, they're definitely worth it.

Not stupid, I also accidentally added something to the wrong container and it took me a while and some searching to find the answer. The feature is great, but the UX is still a little lacking.

The 'Manage Site List' feature is relatively new, it wasn't there in earlier versions.

Also, the UI is... not great. For example you cannot add new urls here. Some websites forward you to some other external url through another domain for example. there is no way to add these custom domains. So annoying. There yet another extension to solve that problem, but... it doesn't support Firefox 'sync'.