solidity – How Opensea Picks Up Base URIs and What Happens When Updated

The uri need to point to a json file.

The json file works with IPFS but over HTTP they usually add the ID of the NFT at the end of the URI which the IPFS Json file will no handle.

Json file = IPFS (one file per NFT) <-- Decentralize

API = HTTP request a DB by NFT ID. (one DB for all NFTs) <-- Centralized

The Json format is used in all cases.

This is the "ERC721 Metadata JSON Schema" referenced above.

{
   "title": "Asset Metadata",
   "type": "object",
   "properties": {
       "name": {
           "type": "string",
           "description": "Identifies the asset to which this NFT represents"
       },
       "description": {
           "type": "string",
           "description": "Describes the asset to which this NFT represents"
       },
       "image": {
           "type": "string",
           "description": "A URI pointing to a resource with mime type image/* representing the asset to which this NFT represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive."
       }
   }
}

From the official ERC721 EIP.

You can also check opensea documentation which is more factual.

You did not set the TokenURI for NFT, so when NFT gets the URI, BaseURI + TokenID is used.Look at the source code.

/**
 * @dev See {IERC721Metadata-tokenURI}.
 */
function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
    require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

    string memory _tokenURI = _tokenURIs[tokenId];
    string memory base = baseURI();

    // If there is no base URI, return the token URI.
    if (bytes(base).length == 0) {
        return _tokenURI;
    }
    // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
    if (bytes(_tokenURI).length > 0) {
        return string(abi.encodePacked(base, _tokenURI));
    }
    // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
    return string(abi.encodePacked(base, tokenId.toString()));
}

It returns the URI: https://gateway.pinata.cloud/ipfs/0

This, of course, is not ideal, the ideal URI should be: https://gateway.pinata.cloud/ipfs/Qmf43nycxRXdwdq3GqT7SKU3MvRyBvbpjzufzZR8ASkMGb

I think you see the reason for the problem here.

The next thing you need to do is call _setTokenUri to store the CID from IPFS so that you can get the correct IPFS link when reading the NFT.

 /**
 * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
 *
 * Requirements:
 *
 * - `tokenId` must exist.
 */
function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
    require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
    _tokenURIs[tokenId] = _tokenURI;
}

This function is an internal function, and you should provide an external function to use it, but security must be taken care of, or anyone can modify it!