There was a time when my understanding was that Zapstore worked on only a singular specific relay. Not sure if that was true or if so, if that has changed since then. It’s a great idea regardless as the decentralized aspect of it unlocks so much. Would it make sense to combine it with Blossom if it’s not yet doing that?

Where does Zapstore keep apk files?

Zapstore is decentralized. You can use it with any relay as long as it follows the Software Application NIP I proposed. To change the relay, at the moment you need to fork the app because it's hardcoded. I will of course make that much more convenient, relay management is coming to the next major version for which I'm working my ass off. Every piece of the stack is open source and I will encourage people to run other relays instances with software releases. I don't know who said it was centralized but why would it be, when it's built on nostr and supported by OpenSats? Makes no sense.