The NFT blockchain is a database in the form of a chain. It consists of digital blocks that are connected to each other like links in a chain: each new one stores data from the previous one. Information from blog
https://merehead.com/blog/nft-wallet-development/ . The platform registers NFTs on the blockchain and takes a commission for it. It depends on the size and quality of the object. To pay the commission, you must have cryptocurrency in your wallet. Some platforms create NFTs only at the time of purchase and deduct a commission from the profit from the sale. For example, OpenSea does this.