Why are My NFTs not found in My Items?
It takes several minutes for X2Y2 to index new items transfered to your wallet recently before they appear in My Items.
How can I know if it's a ERC-1155 NFT?
Navigate to the NFT on OpenSea, click the Details section. There is a Token Standard property which shows whether it's ERC-721 or ERC-1155. Most of NFTs uses the ERC-721 Standard.