How do I find all the ERC-721 tokens held by a certain address? They could potentially come from a variety of sources, not known in advance.
You can check the address at https://etherscan.io/ and see all theERC721 tokens for a specific address.
You can also use the Moralis API to fetch all the ERC721 tokens hold by a particular address. You can look about it here - https://docs.moralis.io/moralis-dapp/web3-api/nft-api#getalltokenids