get https://testnets-api.opensea.io/api/v1/collections
This endpoint provides a list of all the Testnet collections supported and vetted by OpenSea. To include all collections relevant to a user (including non-whitelisted ones), set the owner param.
Each collection in the response has an attribute called primary_asset_contracts with info about the smart contracts belonging to that collection. For example, ERC-1155 contracts maybe have multiple collections all referencing the same contract, but many ERC-721 contracts may all belong to the same collection (dapp).
You can also use this endpoint to find which dapps an account uses, and how many items they own in each in a single API call.