Strongly held but apparently not popular opinion: candidates should not be expected, and should refuse, to discuss confidential internals of their former employers.
Not popular? Who asks someone to break their confidential agreements in front of them, and why would you hire someone who would do that so easily?
Agreed, but what has it got to do with what you replied to?
There's no need to ask about anything confidential. Meta published a lot about their internal tech stacks, and they use plenty of open-source stuff. ZippyDB, Interview candidate can also talk about generic stuff, and I can drill on the theory or common practice.