You don’t do that though. You write your logic in a common languages and then the UI is an external layer you switch with build settings. The thing is, yiur choice is usually c|c++. Or an embedded language like lua. RN has gone the latter route and then extend it further by using React as a DSL for the UI.
> You write your logic in a common languages and then the UI is an external layer
Isn't reimplementing all your UI widgets, views, layouts and interactions for multiple platforms still a ton of work though? I'm not meaning people using things like React frameworks, but the suggestion of sticking to native only.
If you do that, then you intent to take advantage of the native framework (or a mature one like qt for some platform). They are really expansive and you can create an interface very quickly. You usually get animations, themability, navigation,… for free and you’re mostly arranging the layout and writing glue code.
Also, you usually get better tooling in regards to instrumentation and the like.
