It's possible with an enterprise distribution certificate. There are "fake" app stores that distribute pirated apps, they are particularly common in Asia, and I believe many "average" users use them.

Apple can shuts them down by revoking the certificates, but it seems to reduce the visibility, and therefore extend the time that they can work for.

Impossible in the Apple ecosystem without jailbreaking your device. Which hasn't been viable for at least 6 years.

If the code is open source one could compile it and run it directly on their device. But they would need a Mac to build the code on Xcode.