“Sideloading “ is the original app installing by sync or copying.

You used a wire, or Bluetooth that transferred the app file.

Then it ran.

This is how it was.

iPhone 1 was vehemently against third party apps of any kind.

The use of iTunes to have a “store” helped transfer and install apps digitally, and I believe using a wire too.

You either own your device or you don’t.

At a software level mobile has been a challenge to keep secure and locking it all down might not secure it either as there might be side doors still instead of side loading.

It has been 15-17 years since we got this batch of mobile operating systems, maybe we’re due for a new one since there’s a critical mass of users already on smartphones, unlike when Android/iOS began.