For me the way I built the app was crucial. As Josh Morony pointed out here.
If you update something in the code after that you have run the app via XCode on your phone or the simulator, and you want to see the change, you have to build first:
Using Ionic CLI:
ionic build
Using Vite:
npm run build
(npx cap build ios did not work for me)
Then:
ion cap copy ios
ion cap run ios