refactor: add explanatory comments

src/token/erc721/ERC721.sol

@@ -153,6 +153,14 @@ contract ERC721 is IERC721 {
                             INTERNAL CONSTANT FUNCTIONS
     //////////////////////////////////////////////////////////////////////////*/
 
+    /// @notice Returns whether `spender` is allowed to manage `tokenId`.
+    /// @param spender The address to make the check.
+    /// @param tokenId The identifier for the NFT.
+    function isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
+        address owner = owners[tokenId];
+        return (spender == owner || isApprovedForAll(owner, spender) || getApproved(tokenId) == spender);
+    }
+
     /// @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
     /// The call is not executed if the target address is not a contract.
     /// @param from The address that represents the previous owner of the given `tokenId`.
@@ -224,8 +232,8 @@ contract ERC721 is IERC721 {
     ///
     /// Requirements:
     ///
-    /// - `tokenId` must not exist.
     /// - `to` cannot be the zero address.
+    /// - `tokenId` must not exist.
     function mintInternal(address to, uint256 tokenId) internal virtual {
         if (to == address(0)) {
             revert ERC721__MintZeroAddress();
@@ -250,9 +258,9 @@ contract ERC721 is IERC721 {
     ///
     /// Requirements:
     ///
-    /// - `tokenId` must not exist.
     /// - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received},
     /// which is called upon a safe transfer.
+    /// - `tokenId` must not exist.
     function safeMintInternal(address to, uint256 tokenId) internal virtual {
         safeMintInternal(to, tokenId, "");
     }
@@ -303,13 +311,18 @@ contract ERC721 is IERC721 {
         address to,
         uint256 tokenId
     ) internal virtual {
+        // Checks: the `from` is not the zero address.
         if (from != ownerOf(tokenId)) {
             revert ERC721__TransferInvalidFrom(from);
         }
+
+        // Checks: the `to` is not the zero address.
         if (to == address(0)) {
             revert ERC721__TransferToZeroAddress();
         }
-        if (msg.sender != from && !isApprovedForAll(from, msg.sender) && getApproved(tokenId) != msg.sender) {
+
+        // Checks: the caller of the function is authorized to make the transfer.
+        if (!isApprovedOrOwner(msg.sender, tokenId)) {
             revert ERC721__UnauthorizedSender(msg.sender);
         }
 

test/unit/token/erc721/ERC721UnitTest.t.sol

@@ -116,7 +116,7 @@ contract ERC721Recipient is IERC721Receiver {
     }
 }
 
-// A contract with no `onERC721Received` method implementation  that will be
+// A contract with no `onERC721Received` method implementation that will be
 // used for the `safeMint` and `safeTransferFrom` failing tests.
 contract NonERC721ReceiverImplementer {