I'd love to understand what is going on mechanically when these requests are made. Like what's actually being done that takes my google data download request days to fulfill?

My uninformed guess is that it's a service which orchestrates internal API calls to all the other services and builds a tarball. And the reason it takes forever is probably mostly just low priority queuing of all these various requests.

I've been doing this regularly for years, and it's always taken a few days - so I wouldn't describe it as "very fast", but I've never seen it take more than a week.