When you are implementing an NFT, you are likely implementing either the ERC 721 or the ERC 1155 interfaces; as such, certain standard functions will be available using a standard ABI call.
So OpenSea will just try to call `tokenURI`; if you have implemented the interface correctly, that will succeed, if not, OpenSea will not show a placeholder image.
So OpenSea will just try to call `tokenURI`; if you have implemented the interface correctly, that will succeed, if not, OpenSea will not show a placeholder image.