The search for a Sonic Mania Plus Android decomp represents the eternal tension between corporate IP and fan engineering. As of today, you cannot download a simple APK of the decomp. The project exists as source code on GitHub, waiting for a few more hundred hours of labor to fix the renderer and the audio thread crashes.

To run the decompilation on Android, you typically need to build the APK yourself to ensure legal compliance regarding the Plus DLC. RSDKModding/RSDKv5-Decompilation: A complete ... - GitHub

First, we must clarify terminology. If you search for "Sonic Mania Plus Android decomp," you will find two very different kinds of results.