It is important to know that the data.rsdk file often contains the assets for both the base game and the "Plus" expansion. It's not a separate file. In the official game, the DLC is unlocked by a license check. In the various decompilations or unofficial ports, this same content can be unlocked by enabling the dlcEnabled variable.
Always back up the original file. Never share it illegally. And when you see Mighty do his powerful hammer drop, remember—that move exists because of a single line of script buried deep inside the data.rsdk . sonic mania plus data.rsdk