There's an entire ecosystem that surrounds sideloading apps that are not in the app store. They're not all developers or technically inclined people, it's a very easy thing to do and can be done on any OS. Developers can distribute an IPA that isn't valid for your device, and you use an extremely simple GUI tool (Cydia Impactor) to resign, repackage, and install all in a few steps. You don't need to be a technical user to do this. You just sign in with your Apple ID in the app (or any Apple ID, it just needs to be in the program as a free or paid account).