I know Cherubim and some Skitarii are cloned. But yeah they do also use a lot of brains from regular AdMech people for servo skulls, and use criminals' brains for servitors. My understanding is it's an honour to be turned into a servo skull, a bit like getting entombed in a dreadnought.

In general though they tend to use electromechanical solutions first. They only use cybernetics for more complex use-cases. Or for things that require complex sensor input. I've heard it joked before that the sort of automatic sliding door you see at the front of a modern supermarket would be a servitor in 40k, but I don't think that's accurate. They have access to laser rangefinders and simple electronics.

And they do have simple "computers" called cogitators, but these are electromechanical computers like those of the 1960s. They don't use integrated circuits or the like.