To Federate or Not To Federate

Okay, so forgive my ignorance, this is all pretty new for me.

Obviously a lot of us are here as Reddit refugees. People looking for a discussion forum that scratches that itch. Fediverse sounds pretty neat. Only I'm not really sure what to make of these instances. Basically, I need a separate account for each Lemmy instance? So if for some reason .world doesn't have what I'm looking for, I literally need to make a new account on someone else's instance? For example, my login doesn't work on lemmy.ca. Isn't the point that you can comment on anyone's post no matter what because it's all run through Activity Pub? Or do I have to make my own instance to federate other people's instances? Super weird.

Am I missing something here? I just want to have a place where I have 1 login and I can see the entire internet's worth of interesting stuff. Like Reddit.