I wrote my first Android apps using an emulator. Then used the proceeds from selling those apps to buy my first Android device and a better computer.

(Doesn't mean that they have to, of course. Just that it's possible.)

> How are they suppose to write an app when they have no phone?

Android and iOS both have emulators. React native/expo can run in browser, etc.

> And which AppStore? Apple, Google, Sourceforge?

Seems like that's missing the point

The App Store.