In my initial talk with the recruiter, they mentioned the procedure would be an OA (or phone screen coding interview, my choice) followed by onsite.
I finished my OA and the recruiter replied within hours that I'm good to go for onsite. However, the recruiter asked if I'm free for a call to go over my experience, CS fundamentals and system design with them for 30-40 minutes.

Has anyone had this before? That seems like a pretty short amount of time to go over all of that in depth. And I've never heard of a tech screen done by the recruiter themself. Is this common in Amazon interviews? I've seen a couple other leetcode posts mention it. What kind of things do they ask as "CS fundamentals" ? Pretty vague so I'm a bit anxious.