【solidity】ERC-20準拠のトークン発行やスマートコントラクトをより簡単にするライブラリ「OpenZeppelin」について

pragma solidity ^0.8.9;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";//①
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";//②


contract MemberNFT is ERC721 {

  //トークン名とトークンシンボルを定義
  constructor() ERC721("MemberNFT", "MEM") {}


 ////この記事では、この関数に注目してください////////////////////////////
  function nftMint(address to, string calldata uri) external onlyOwner {
        _tokenIds.increment(); //0を1増やして「1」にする
        uint256 newTokenId = _tokenIds.current(); //さらにここで１増やす


      ★  _mint(to, newTokenId); //NFT作成  ①の中に定義あり
      ★ _setTokenURI(newTokenId, uri); //newTokenIdにuriを紐づける  ②の中に定義あり


        emit TokenURIChanged(to, newTokenId, uri);
    }

pragma solidity ^0.8.0;

import "./IERC721.sol";
import "./IERC721Receiver.sol";
import "./extensions/IERC721Metadata.sol";
import "../../utils/Address.sol";
import "../../utils/Context.sol";
import "../../utils/Strings.sol";
import "../../utils/introspection/ERC165.sol";

contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
    using Address for address;
    using Strings for uint256;

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

    // Mapping from token ID to owner address
    mapping(uint256 => address) private _owners;

    // Mapping owner address to token count
    mapping(address => uint256) private _balances;

    // Mapping from token ID to approved address
    mapping(uint256 => address) private _tokenApprovals;

    // Mapping from owner to operator approvals
    mapping(address => mapping(address => bool)) private _operatorApprovals;

    /**
     * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
     */
    constructor(string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }


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//以下、関数がたくさん定義されている。ここでは「_mint」関数のみ抜粋

    function _mint(address to, uint256 tokenId) internal virtual {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _beforeTokenTransfer(address(0), to, tokenId);

        _balances[to] += 1;
        _owners[tokenId] = to;

        emit Transfer(address(0), to, tokenId);

        _afterTokenTransfer(address(0), to, tokenId);
    }

pragma solidity ^0.8.0;

import "../ERC721.sol";
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    function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
        require(_exists(tokenId), "ERC721URIStorage: URI set of nonexistent token");
        _tokenURIs[tokenId] = _tokenURI;
    }