@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!
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.