> So Android allows alternative rendering engines besides Chrome for PWA?

Yes: https://developer.mozilla.org/en-US/docs/Web/Progressive_web...

I tested just now in Firefox with an app from https://appsco.pe and it does indeed work!

I can do the same with the Android version of Brave.

> If you install Firefox it uses Gecko but still has native app look feel?

That depends on your definition. Making an app _feel_ native is a matter of implementation. But the opposite is also true: A native app is free to feel non-native if the app creator makes it that way.

The app does show as a distinct entry in the app switcher, but still has a Firefox icon when I tested it just now.