I’ve never seen a mounted drill in the OR (though I imagine there is an option for it - bed-mounted instruments and equipment are pretty common).
Here’s a video that kinda shows how craniotomies go - this is just an animation, nothing gory. The drill in the animation is different from the onces I’ve seen used for cranis (pistol-shaped vs just a cylinder like the one I linked earlier) but either way, it’s very much a hand-held device.
Even micro surgery like when we’re drilling in a tympanoplasty or cochlear implant placement - literally done under a microscope - it’s still just a little dremmel looking thing.