The levels, music, and skins are all NTFs.
The owners of the NFTs can issue them in several different ways.
If the owner wants to create rarity they can create a fixed number of the NTF and auction it to create exclusive content.
The owner could mint infinite NTFs at a fixed price... or maybe only sell them for x amount of time.
The owner could also just "open source" the NFT and make it free for everyone for reputational gain (tips allowed).
The base game would probably come with several free/default options.
The content creators would then be engaged in free-market capitalism/competition with one another.
The interesting thing about these NFTs is that they are actually fungible within the context of cloning multiple ones and they have all the exact same value and utility as any other clone. So maybe NFT isn't the best word to describe them, but now that I mention it I have written a post called Fungible NFTs.
Another term for this is "fractional NFTs".