Fly from Xiamen to Los Angeles and pick up Charlotte. Where did it go through customs?

Xiamen flies to Los Angeles to transfer to Charlotte, and Los Angeles passes through customs. According to Baidu map, because you need to transfer in Los Angeles, Los Angeles Airport has facilities of US Customs and Border Protection (CBP), which is responsible for your entry inspection and security check. Therefore, you need to go through the customs at Los Angeles airport so that CBP officials can check your identity and travel documents.