Relative post:
https://forum.unity.com/threads/using-unity-as-a-library-in-native-ios-android-apps.685195/

To reproduce:
1. Go to <YourEditorRoot>/Contents/PlaybackEngines/AndroidPlayer/Variations/il2cpp/Release/Classes
2. Open the classes.jar file with compatible software (example: jd-GUI)
3. In jar file, go to com > unity3d.player and observe UnityPlayerActivity.class missing from the classes list

UnityPlayerActivity as a .java file is now located in: <YourEditorRoot>/Contents/PlaybackEngines/AndroidPlayer/Source/com/unity3d/player
Currently, the editor cannot be extended by the means that are mentioned in documentation: https://docs.unity3d.com/Manual/AndroidUnityPlayerActivity.html

WORKAROUNDS for creating plugins in Android Studio:
• Getting classes.jar to your Android studio project from older Unity editors (anything older than 2019.3.0a2)