It's confusing, I know 😅 You use one account to log in to a platform you like and it should[¹] give you access to everything else through Federation.
[¹]: ActivityPub is the underlying protocol used by the majority of federated platforms, so it should work albeit on a basic level, because some platforms don't support advanced text-formatting, others are arranged in threads, etc. so, YMMV.
For this reason, I find it useful to have one account per platform.