Questions about "in house" movements

[csaltphoto]

What exactly does it mean, at least as pertains to Rolex? I know the difference between say ETA and a modified ETA. And I know that some parts of a Rolex are outsourced (At least the handset and mainspring, probably some small parts like screws and such). I'm sure they don't actually manufacture their own synthetic rubies...

I was looking at the Esslinger website the other day. And they have "generic" Rolex movement parts. Nothing big like the mainplate or the rotor but a few gears, screws, jewel bearings, that sort of stuff. Mainsprings of course.

Several reasons I am asking. I hear these stories about Rolex refusing to service a watch because it has non-Rolex parts and I hear stories about Rolex insisting on replacing non-Rolex parts if encountered during a service. I mean if the case or mainplate was inauthentic I can imagine them refusing service. But can they really authenticate every single part? Is every screw and gear somehow identifiable? Are these generic items the same as Rolex uses but just available because it's more cost effective (or even qualitatively better) to outsource their manufacture to a specialist?