Exchanges and 3rd party wallets who manage their own UTXO’s often use a local block explorer like cardano-graphql or cardano-rosetta .
cardano-graphql is a Cardano query language for API and a runtime for fulfilling queries with existing data extracted from the cardano-db-sync PostgreSQL database. It provides a complete and understandable description of this data and gives clients the power to request the necessary information. This, in turn, makes it easier to evolve client API’s overtime and enables powerful developer tools.
cardano-rosetta initially developed by Coinbase as blockchain middleware, has been adapted by Cardano to help exchanges and 3rd party wallets extract blockchain data and integrate with Cardano. For more information on learning rosetta see here and extracting multi-asset information from cardano-rosetta please visit the cardano-rosetta multi-asset Readme