Zeauro already explained the view layer/collection system pretty well, but just a quick tip if you hadn’t noticed: the closest thing to 2.7’s “jumping into an empty layer” would be to create a new collection, and ctrl-clicking the eye icon, which hides all other collections:
Another ctrl-click unhides everything again. This takes a few clicks (you need to remember to activate the new collection before adding objects) but at least it’s much quicker than opening up a new file.
I was first annoyed that it unhides all collections with the second ctrl-click (even ones that were hidden before the first click), but when you start disabling the collections you want to be hidden, it’s not a problem anymore.