I have not coded any mobile apps, I understand the downsides of using react native/electron due to them being web apps but what about things like Kotlin multi platform or flutter? Aren't this frameworks supposed to allow your to have 1 codebase and the apps be truly native instead of web apps?

I think any framework that aims to support desktop and mobile frome the same code per view/screen is doomed to either make one of those two awkward to use, or alternatively become itself very complex to use.

The most advanced in this area may well be websites and into those decades of work of thousands of engineers went (HTML, CSS and JS standards).

React Native apps are not webview based like Electron, they use native components. Arguably it is more native than flutter, which uses its own components designed to mirror native ones on Android and iOS.