Your profile is like a tiny file of only your most basic info. It exists on many relays. Firstly, your write relays. When you change your profile information, the old copy is discarded, replaced with the new, and broadcast to many relays that do the same discard/replace procedure. When you log into a client, it goes searching the network for the most recent copy of your profile that matches the key you provided. I kind of think of npub as a file name & nsec as the key to access the file, if that makes sense. That's probably not quite accurate, but it's the best analogy I've got.

The more you interact on the network, the more relays end up with a copy of your profile. Things work more smoothly if you use nostr regularly. Relays are just servers run on individual computers, whether it's me with my little vps or a big company's servers, so no one is beholden to keeping your info indefinitely. Old data, unaccessed gets discarded & such