Please note that while you can add NFT's as custom tokens in the extension, you will not be able to see them natively in the UI.
MetaMask mobile displays NFT's under Collectibles. Please remember that some NFT's will not show up on the mobile app under Collectibles, or may not show up correctly.
We pull data from opensea.io, it's our 3rd party repository for all NFTs. A collectible ID is a unique identifier since no two NFTs are alike. Please try finding the ID with the app where you purchased it.
NFT token 1155 is still in development, our product team is planning to add it sometime in Quarter 4 2021. Please follow our community for any updates.