How does turnitin work?

So recently we all got our first grades back and after a talk with my classmate he said he literally copied and pasted from chat gpt and passed.

I didn’t believe him as we all sent our work via turnitin and obviously it wasn’t flagged. He then told me that teachers don’t scan for ai for all students, only the ones that they suspect have ai are scanned as it would be expensive to scan all students work.

There is no way that’s true. Right?