At the moment iOS is the limitation here, how link code together (However I think Jetbrains work on fix this). Hence right now need one shared module between iOS and Android. The folder structure doesnt really matter, but easiest create them as siblings imo. In Android/Gradle there is no limitation in amount of modules. Myself prefer using feature modules, which is in KMP used by the shared module.
So shared module entry point for all app folders like iosApp, androidApp, desktop app, webApp etc.