Unlike standard Android apps that come as a simple .apk file, Retro Engine games (like Sonic Mania) separate the game logic from the game assets.
Are you planning to use any or looking for help with the APK building process ? RSDKModding/Sonic-Mania-Decompilation - GitHub sonic mania data.rsdk download android
: Copy the Data.rsdk file from your PC to the RSDK folder on your Android device. The Ultimate Guide to Sonic Mania Data