I've seen some ios apps that can be downloaded directly from their websites, then you have to go to settings and trust the developer.

How can this be achieved?

CodePudding user response：

In order to distribute an app outside the AppStore we need to fall back on something called internal OTA distribution (Thanks to Matt for this information!)

OTA distribution can be achieved by sending device UUID to developer that can then generate a new profile that will be associated with the app to enable it to run on your device. For this kind of distribution, app duration is very limited on time. TestFlight is another resource that can be used to distribute an app outside the AppStore. It enables your user to use the app for 90 days. (Thanks to Ptit Xav for this!)