Mastering Blockchain Programming with Solidity

Token owner mapping kept in _tokenOwner

The mapping is used to find the owner of a given NFT ID. In the code, it's defined as follows:

mapping (uint256 => address) private _tokenOwner;function ownerOf(uint256 tokenId) public view returns (address) {    address owner = _tokenOwner[tokenId];    require(owner != address(0));    return owner;}

_tokenOwner is a mapping from uint256 to address, which stores the mapping for an NFT ID to the account address of the owner. 

The mapping variable is private, and the ownerOf() accessor view function fetches the data from this state variable. The ownerOf() function finds the current owner of the given tokenId.