Basically you want to share the business logic between them. You still need to spend time to write UIs for iOS and Android which following interaction paradigms of the platform.
Personally, I can't stand iOS apps that look like Android.
Currently, I am happily working on a project where I write all the business logic in Swift and share between the web app (WebAssembly) and the iOS and Android app. The only bit that I customise is the UI part. I quite enjoy this approach.
(I have to admit for me writing native mobile app in JavaScript/TypeScript feels wrong. Guess, I don't like that language enough)