I'm working on an NFT project, Once I created an item on Rarible, and after that login on to Opensea with the same wallet address. Then the item I was created on Rarible is automatically synced to Opensea including title, history, etc. So, As a programmer. I want to know the reason behind this process and how they are getting this data from one to another using the same wallet.
When any contract writes an ERC721 compliant transaction to Ethereum, marketplaces like OpenSea and Rarible are able to monitor new blocks (e.g. via event streaming) for new token creation. Since this information is public, information such as the issuing contract id, owner address and token id are immediately available and can be retrieved. Additional information about the contract can also be obtained from the contract constructor.
Additionally, one of the methods on an ERC721 contract is tokenURI. By navigating to this URI (often stored on immutable storage like IPFS), anyone observing the blockchain can retrieve additional metadata information about the token. This metadata usually includes an image, description and other custom attributes which describe the NFT. This allows marketplaces to render the NFT on their exchanges.