You only need one account and that will allow you to post and comment on other instances.

You can use your account to comment and post on communities on other instances. This does NOT mean your login will work on their site.

If you go to https://Lemmy.ml/C/mildlyinfuriating@lemmy.world you are now interacting with a community on that instance. The posts, comments and votes are synchronized across at a server-to-server level and cached in Lemmy.ml

Your account is just on the instance you created it so you can’t use it on other instances but you usually doesn’t need to … you can follow communities from other instances without having an account there.

You can’t, but there’s no need. You can read posts to communities on other Lemmy servers from your account on lemmy.ml. You can subscribe to those communities and comment on posts there as well. There’s no need to have an account on each server. It’s like email. You don’t need an email address on each server to send/receive email. There may be a reason to have another account - maybe you get really involved in a community on that server and that server feels more like home. But, no, there is no protocol for cross-authentication.

It’s really a lot like email, only the messages are (mostly) public instead of private.

If you have an account at gmail.com you can send and receive emails with someone at another site like mit.edu, because your mail server and their mail server know how to talk to each other (that’s “federation”).

But you don’t have to personally log into their mail server to do that. You just use your account on your server, and the servers take care of getting messages to the right server for the recipient.

Similarly, here, you can subscribe to and comment on forums (“communities”) originating on other instances. You don’t have to personally log into the other instance to do that; the servers take care of that part.

“You can use your account you created to log in to the server on which you created it. Not on other servers. Content is federated to other servers, users/accounts are not”

That’s not how it works :) Your lemmy.ml account is tied you this specific instance.

IIRC they are looking into an option to transfer your account between instances but I might be wrong.