To add to OP, Nick Craver (formerly @Stack Overflow) wrote an insightful series [1] (2016) on Stack Overflow’s architecture.

[1] https://nickcraver.com/blog/2016/02/17/stack-overflow-the-ar...

Popular as it is, stack overflow is a very low volume website by modern standards. In fact it is very successful by introducing rules to throttle user input( thus exposing more filtered info to the visitors).

It's still way more volume than the vast majority of engineering playgrounds out there. I've seen stacks orders of magnitude more complex (with corresponding cloud bill) that served dozens of requests per second at most.