No, your account is tied to the instance you signed up for. However you can browse and subscribe to communities outside of your instance. For example, you can search for asklemmy@lemmy.ml from the beehaw site, and subscribe to their updates with your beehaw account.

A way to think about it is your account is owned by one instance, including your profile and subscriptions. Instances host the actual content.

Note that the instance must permit the content from the other instances (i.e. “federate” the content) for it to be searchable. This is why there may be missing content from select instances / communities (in other words, your instance explicitly blocks the content by choosing to not federate with them).

You can have the same name in different instances, since it is actually tied to your instance (e.g. mine may show up as rknuu, but it really is rknuu@beehaw.org with the instance name appended).

From my understanding, no. You can comment on and up vote content outside of your home instance, though. But channels and usernames are not unique between instances.