When a “custom Android build” gets created from the Godot android export, the generated GodotApp contains references to a GodotLib, which is provided as an AAR in the project.

A quick look at it in Studio shows references to a lot of stuff like event management, but also Payments classes, etc.

So let’s look at what Godot has that is linked to Android:

https://docs.godotengine.org/en/stable/tutorials/plugins/android/android_plugin.html: A “Godot Android plugin” is an independent AAR that one can add to a Godot project, and then use from the GDScript.

For example there’s a plugin for Oculus VR, and an official one for Google Play Billing compatible starting with Godot 3.2.2.

GodotPaymentsV3 is an older library that is superseded by the above Play billing plugin.