The best way to explain the Omnissiah is like this; take a crotchety old truck that never wants to start, so you have an almost-superstitious ritual of tweaking the choke just so, jiggle the key a certain way to get it to slot in all the way, and then step on the gas exactly 4 times as you crank the motor in order to get it to fire up. In the 40k universe, the Omnissiah is the force you're placating with that ritual, and if you don't, the capital-m Machine will not obey you.

tl;dr- all machines in 40k act like printers.

There's a bunch more behind the Omnissiah / AdMech storywise, but that would be getting wayyy off track

"This is probably a stupid question, but . . ."