I'm a Program Manager (PM) for one of Those Big Companies. I don't actually program except as a hobby, and I don't manage. Where I add value is making sure that what we're building is what our customers need.
"I could build a replacement with just ___ people"
No, you couldn't, if it's a complex project. I was recently reminded of the true history of the automobile: a long, hard slog by an enormous number of people to get each element of a car to actually work. In particular, take a look at Motor-Car Principles on Project Gutenberg. Among the many, many things that had to be painfully figured out
How to stop a car. The book lists a whole series of brake types (none of them disk brakes like in modern cars)
How an axle works. Car axles are not wagon axels, and you can't use one instead of the other.
Building a transmission. This is something steam engines, for example, simply don't have. A steam engine on a train is directly connected to the wheels with no intervening transmission.
Making a piston. Steam engines, in contrast to car engines, are forgiving: as long as the timing is roughly correct, the steam engine will work. A gas engine has much higher tolerances.
The book goes on with a long, long list of technologies that are unique to cars.
Moral of the story: some projects are pretty simple and can be done with a small team (or even just one person). But once you get to new technology, it's a quagmire that requires an extraordinary amount of work just to know what needs to be done, and from there to co-ordinate the work so that all the parts show up on time.