Yeah good point. They used to use it and then had to swear off it. It's a common sci fi trope. See also the Butlerian Jihad in the Duneiverse

Haha yes, no AI in the Imperium, no sir, just machine spirits, the chosen of the Omnissiah!

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.

"Build routine seven two one initiated."

The Machine Spirit is (mostly) an abstraction. The AdMech as a whole is a collection of tech support geeks whose documentation and manfiles were converted to hymns 10,000 years ago. Very few members actually have any significant understanding of what they're doing anymore. They just know that if they speak the Holy Words of System Reset and then poke the button with a circle on it, the machine spirit dies and resurrects anew.

So, mostly, no, AI has been well and truly eradicated in 40k and most of what is referred to as "Machine Spirit" is just semi-basic technology being crabby like it already does in the modern day. When a modern programmer says his system is "thinking" or is "upset" he doesn't mean it literally - AdMech just forgot that language wasn't literal.

That said, there are a few suspiciously intelligent and well-informed Machine Spirits in a few places. A couple of them can outright hold a conversation.

There is, actually. They are one of three things:

AI, VI, Warp Spirits.

Plenty of machines have AIs that are either hiding, hibernating, or degraded. There are also plenty of VI programs that are just very complex and sometimes will function in ways their users don't understand.

The rest... Well, belief matters when your species is made up of latent psychics. The best way to get a helpful machine spirit is to treat it with respect, so a decent number of machines that had VIs become something more with time. Not only will it empower the spirit, it will prevent it from falling to Chaos...

In theory.

In my mind that’s what they were, though as a kid I had clippy in mind instead of Siri

Well, no. The Imperium places the Emperor above all else. The Adeptus Mechanicus is the only part that idolizes machines. The rest idolizes a corpse on a toilet.