At the moment they work like email addresses. You have an xyz@gmail.com account, you can only log into gmail.com’s server. But you can talk to abc@outlook.com, the fediverse (gmail.com and outlook.com’s servers) takes care of passing the messages / posts for you.
@ryujin470 You can't log into other instances from your Fedia account. Not yet, at least. I know that account portability is something that many Fedi platforms are working on improving, so that could change in the future!