Fediverse

Not necessarily. You can subscribe to a specific community on another instance by searching from your "home" instance with !<communityname>@<server> and subscribing from there.

If the instance is being federated (i.e. it's content is getting pulled in by other instances) then no. If you really want to participate in an instance that's effectively been blocked then yes, you'd need an account on that specific instance.

You don’t have to. Implementation is a bit buggy but in theory you should be able to subscribe to and view all content from federated instances from your “home” instance

Think of it like an email address. Any account you have can talk to any other email provider, there are no limitations on that.

If you are not happy with your email provider (Lemmy instance), you can always create a new address at another provider and use that one. There are currently no features to automatically migrate all your history, but that may come.

Nope. You have access to everything from just one; just search for the community in the search tab and join it

To go there directly from the address bar, put /c/[email protected]

I think it would help if you share what specifically you want to do on other instances.

Just follow communities on them? No need, you can follow from where you already are, like the other reply says.