Each account is unique to the instance you signed up with, it's the content that is federated. For example, I'm replying from lemmy.world.
Fediverse
This magazine is dedicated to discussions on the federated social networking ecosystem, which includes decentralized and open-source social media platforms. Whether you are a user, developer, or simply interested in the concept of decentralized social media, this is the place for you. Here you can share your knowledge, ask questions, and engage in discussions on topics such as the benefits and challenges of decentralized social media, new and existing federated platforms, and more. From the latest developments and trends to ethical considerations and the future of federated social media, this category covers a wide range of topics related to the Fediverse.
As far as I am aware, logins are not federated. I would think that federating the logins would give opportunities for other instances to potentially decode your password because of the way that federation works.
You are attempting to go to other people's homes to watch their tv, featuring the tv channels you already have at home. You are struggling with your house key not working on their doors.
I recommend you stay in your own home and surf the channels from your own tv.
Meaning, add the subscriptions from your own kbin server, to their content.
You don't need to go to other instances too see their content. You'll see it cached on your own.
If you click on communities (magazines?) And select All you'll see some from other servers.
There are community explorers that can help you find communities in other instances (like lemmyverse.net)
If you find any there you like but don't appear in kbin, you can open them and instead of a suscribe button they'll have instructions on how to add them to your server