I understand its a hugely popular service and there is a lot that goes in to something so big but there are bigger websites that run on much much smaller teams

I understand its a hugely popular service and there is a lot that goes in to something so big but there are bigger websites that run on much much smaller teams of developers. I just don't understand how there is even enough surface area on the app that 1000 people could be working on it at the same time. Is that number counting people working on the ops stuff or building other tooling not part of the main app?