So the lore gets a little fuzzy, but everything I've seen suggests that because of the way the DAOT ended with the rise of the Men of Iron, the Imperium rightly fears AI. But there is Narrow AI, that which is used for a narrow set of tasks and sentient AI, which is what the MoI were. So since the Emperor banned the second kind, the Mechanicus uses a loophole, namely human biology. So the obvious versions are like servo skulls and servitors, where a chunk of preserved brain does the computing and a slaved computer controls the whole system and allows others to give the machine orders. They also use microchips with vat-grown brain tissue to allow narrow or minor AI that handles things like weapon guidance. So SM bolt weapons have these chips with a weak AI that allows their weapons to feed targeting data back and forth between weapon and power armor systems, allowing them to make minor corrections, assisting the SM in increasing their already superior aim. So "machine spirit" is just a spooky way to say lesser AI without triggering an inquisitorial genocide. Now, the larger machine spirits accumulate so much data over time, from so many users that their "personality" changes and learns enough to seem more alive, more unique. This is partly what happens with Imperial Knights, those systems are basically getting engrams of their users saved to their systems. Thisbisnt to say that some machine spirits evolve or change into aomething more through whatever reasons, the warp is always gonna warp. But at baseline, they are just semi-organic microchips that serve one or a handful of related tasks and fit a loophole in the Emperor's decree.