Usernames are unique only to the instance you registered with. Someone can be CrimsonFlash on a different instance than the one you are on, just like they could be CrimsonFlash@gmail.com or CrimsonFlash@yahoo.com.

There’s no need for you to register at all the instances, just pick one and follow the communities you’d like to.

because each instance will append it’s address to the end of your username, it technically is a different username per instance, for example:

@brunofin@beehaw.org -> that’s me on beehaw, where I am writing from

@brunofin@lemmy.ca -> that would be me on your instance but I don’t have a user over there

@brunofin@mastodon.social -> thats actually me on mastodon, which is another instance of another service in the fediverse.

it’s unique to the one you signed up with.

Yes, and if you are using the android app it lets you switch by showing you the instance names as part of the user names. Although it doesn’t show the instance for the currently active account.