You in fact, did not register for an app. What you (or well, the app did this for you) did was randomely picked a number an kept it a secret; you then cryptographically derived another number and made it public as if its a 'name'; you then wrote a publication under that public number and added a cryptographic signature based on the secret number; you then send this publication to a bunch of servers (we call them relays) making it available for others to retreive and see it.
One of those publications is you declaring you go by the name Demi and what profile picture and bio you use; another publication was this introduction post; and all the replies you gave thusfar.

Because your identity is based on this pair of numbers (what we call a secret and public key) that you controll, you are not stuck within an app, you can use that same pair in a wide variety of other apps, apps that can have a wide variety of purposes and features;
Because your publications have a signature only you can produce, those publications are not stuck on one server, but can be hosted by whatever relay out there, both now AND in the future, whilst your audience can verify it came from you, regardless where they get the publications from.

Welcome to a new paradigm, stick around and you'l figure it out