You can't install iPhone apps from outside the App Store on an iPhone (unless it's rooted, which most people prefer not to do for reasons of complexity and security).
I'd say the Android app store is a nice balance.
Also, if you want to install some other, less-well maintained app store on your Android, that's also possible (on non-rooted phones, too). Indeed, I've got F-Droid enabled on my Android.
That said, if Google ever pulls the ability to sideload apps on non-rooted phones, I'll not be happy at all.
I'm surprised Google didn't pull the ability to download an APK and install it from Android Q. They modified the permissions slightly so now you have to click "allow" again every time an app wants to install an APK it has downloaded. Up until Q, once you used an app to e.g. update itself, it maintained the ability to download and install updates forever.
When Fortnite was distributed outside the Play store I thought that would be the catalyst for removing side-loading permissions, only retaining them over ADB so developers would not be totally screwed. Maybe the news of that arrived too late in the Q cycle for it to be implemented. I'm sure that one day on-the-fly side-loading is going away. Maybe not in Android Q, but in R or S definitely. It's too consumer-friendly and useful! Removing it will "help user's security" while at the same time removing the ability to install 3rd party VPN-faking ad-removers or games like Fortnite that don't have to pay Google 30%.
