It is possible to run a Bitcoin miner, which will only be cost efficient with a really nice GPU. But mining Bitcoin only earns money by appending a transaction at the end of a leger moving Bitcoin into a wallet. Even if you have a wallet you will eventually want to turn that into real money. Those exchanges require an account with them tied to your real identity. The transaction can then be traced back to the wallet where the miner deposited the coin. There may be some hoops you can jump through, but the leger being public makes it possible to find crooks when they try to cash out.

Push notifications.

It is fine hosting a service that gets requests then talks to FCM or the iOS version. But a service that one’s phone stays connected to 24/7 is really hard, and not kill one’s battery.