For web apps, you can self-host.

For Android, you can install the apk without an App Store — you just won’t get push notifications.

For iOS, you could build the code and install on your own device, which is a step more difficult. There’s rumors that sideloading apps without the App Store will be allowed in the near future due to EU regulations. If and once that happens in the way that people hope, it might no longer be centralized.

For relays, each individual relay is indeed “centralized” in isolation, but the fact that a client can connect and broadcast to many means the system as a whole is decentralized.