Beta & Nightly builds
OsmAnd produces every day multiple Ready-To-Use builds which allows keep track of new features and participate in OsmAnd contributions (translation process).
Google Play Beta builds
You can participate in testing OsmAnd beta build thru Google Play.
OsmAnd-default.apk — nightly version of OsmAnd free.
iOS beta builds(TestFlight) usually are provided weekly (after team sprint demo). They don't have description and could have continuous versioning. They supposed to be stable and ready to use.