Data is stored in relays. You should be able to see which relays you’re connected to. You can add relays, remove relays, or even run your own and store your own data.
Discussion
So the shelling point is the OS, in terms of its censorship resistance. Delete the app from App Store and the laymen has no access? Is there a fix to this?
You don’t need an app, you can just browse on the web as well. Walled Garden OS are a burden but not really pivotal to Nostr.