NFT stands for “non-fungible token” — “non-fungible” means it cannot be exchanged for something else. It is, by definition, one of a kind and un-alterable. For example, you can’t exchange an image by Sally Mann for another by Ansel Adams. They are not the same thing.
NFTs are verified using the blockchain, which is a transparent history of ownership, purchases, and trades that no one can edit but anyone can see. It is a decentralized ledger spread out over computers worldwide. The blockchain ensures there will always be a trace from the current owner back to the original creator.