TestFlight is Apple’s process for distributing beta apps on iOS. Developers can have a limited number of users register for the beta, and then users download the beta app through the TestFlight app.

GitHub is a Microsoft-owned platform where developers store their code repositories. I don’t have an Android device anymore, but I think users can download a packaged APK file from GitHub to directly install a non-PlayStore app. You’d probably need to update your settings to allow installs from unknown source’s though.