How bitcoin application in production retrive UTXO's
Most services operating large scale wallets only use Bitcoin Core for networking information, and maintain a separate utxo set for wallet purposes.

There are many existing options to build a full utxo set that can be queried for arbitrary addresses, such as the insight project.

Moreover, recent versions of Bitcoin Core come with the scantxoutset RPC call, which allows you to locate unspent outputs for a given address or locking script.