https://www.theverge.com/platform/amp/2020/8/13/21368079/for...
But I do wonder, can and _would_ you do payments in a non store app with their own payment provider? E.g. the fortnite you have downloaded and installed outside of the play store?
There won't be anymore than what the web already has. Stripe or Paypal would likely be popular choices. Payment processors are required by law to be PCI compliant, though that doesn't guarantee they are.
Yes I would do payments in a non app store app with their own payment provider if I felt I trusted the app. It's no different than paying on any website (there's no Apple review process for websites).
At this point it's just like any software on your computer, you can pay outside of it, and use a code or similar to indicate you paid for it.
It's only a matter of convenience to want to pay in-app. It can also be a lock-in strategy from the developer - and this is what this whole thread is about.