On Google's android, everybody uses Google's browser. On Apple's iOS, most everybody will use Apple's browser.
Don't forget about Google search which will push Chrome every chance it has. And Youtube.
Which cannot be said about Windows.
In Android, the app just throws an intent to open something, the OS opens the list of app capable of handling it, the user picks the wanted one (either 'only this time' or 'use this as default next time'). The OS, not the app, then remembers the choice.
I don't think they are responsible, but you can invoke a specific app if you need. Google's apps keep showing this sheet with "browser choice" quite often, conveniently forgetting that the user keeps setting "don't ask me again".
The choices are: Chrome, Google, Safari, Default system browser.