Non-GamStop operators are universally browser-based (no App Store presence due to gambling restrictions). The mobile-web UX varies hugely. Tenobet has the cleanest mobile-web UI in my testing. MyStake second. Goldenbet feels desktop-first and laggy on mobile.
Tenobet's PWA install option is the closest thing to an 'app' from a non-GamStop. Saves to home screen, works like a native app. Worth setting up if you play on mobile primarily.