Neither Android nor iOS permit sideloading of iOS apps. Android doesn't help much if that's what you want to do.
Uh what? Every Android phone lets you install apps via .apk files.
How do you get them onto the iPhone tho?
How do you turn an iOS app into an apk file?
Fair, guess I missed the part where you said "of iOS apps". Then again, very few apps are iOS/Android exclusive, so most of the time you can just sideload the Android version of the app.