I want to combine CodeQL with CPG, so I would like to know the principles of CodeQL when performing queries and which interfaces and methods are called during the query execution.

Have you read the codeql language reference and the codeql capture the flag exercises?

Thank you for your reply. yes, I've read QL language reference recently, and I've written a few queries and checked the query logs, but I don't have any idea of using codeql ql to query CPG(Code Property Graph), what should I do?

We don't have any public documentation on how to create an extractor for a new language. Though, I'm not exactly sure what you are trying to do. It sounds like you have Code Property Graph data for a program that you want to add to a database? Which language are you working in?

I want to store CPG of multiple languages into the database and then perform queries through QL. I came to know that due to the unavailability of the parsing engine part as open source in Codeql, this task can not be achieved.