[qqii]

Current NFTs depend on smart contact platforms.

A token contract (or any contract) is unique by address. To simplify the ethereum nft standard specifies that the token can only have one owner. This is what is called an NFT.

NFTs don't need smart contacts, they could be part of the protocol. As an example you can create a Cardano token with a maximum supply of 1.

An external representation or as you call it "metadata" only serves as a representation of the token contract address (and therefore creation address) and state of the contract which returns your public key as owner.

Unless the "metadata" is another NFT it cannot be unique by definition - digital uniqueness relies on ownership by asymmetric key cryptography.