esmCan I log in to any instance with one account?
ArtemisDownNo, you have to make them per instance, but its sorta redundant as each instance communicates with each other and displays the same content / can communicate with each other (example: You're lemmy.sdf, The user below me is Beehaw, and I'm shi.tjust.works
That's mostly true, but there are a few exceptions where certain instances choose not to federate with certain other instances. It is also possible to make an instance that does not federate at all and therefore would be locked to that single instance.
Im still new to this but can you still manually subscribe to a community of the "banned/unfederated" instance if you have the URL?