Vitor is right when he says you don't want an iOS-looking app on Android and viceversa.

Flutter goes a long way to give you base themes for Android (Material) and Cupertino (iOS) both of which are very customizable. I do not know whether RN has an equivalent but he put both in the same basket so I'm clarifying.

As long as you respect basic HIG for each you are fine, navigation and inputs in particular. The latest iOS glass effect? Who cares. We got bigger fish to fry.

The world is going to the shitter and we need more apps out there faster.