How does pedaling power transfer to the bearings in Hollowtech II bottom bracket? I just installed a Shimano Hollowtech II crank and bottom bracket on my bike, and I'm wondering how they work together. 

My understanding is that the bearings are located in little cups that screw into the outside of the bottom bracket shell. They are sealed but can spin freely. To install the crankset, first you insert the pedal spindle (with drive-side crankarm already attached) through the bottom bracket shell. Then you attach the other-side crankarm to the pedal spindle via a splined interface and then tighten its two bolts. That much makes sense.

But how does crankarm-spindle-othercrankarm assembly interact with the bearings? In particular, when you apply power to the crankarm (via pedaling), how does that power get transferred to the bearings? The outside of the bearing housing is flat (see first photo below) and I can't see how the inside of the crankarm would interlock with the outside of the bearing housing. The inside of the bottom bracket tube is smooth too, and recommended to be greased up. It seems like the only thing connecting the crankarm to the bearing housing is friction, and that seems like it would slip and be generally unreliable. The only thing determining how tightly the crankarm is set against the bearing housing is how much you tighten the other-side crankarm with the "bearing preload cap" (see second photo below) but the bearing preload cap is plastic, kind of flimsy, and explicitly recommended to be not very tight. 

Can anyone explain how this all works together?
You seem to be assuming that "power has to be transferred to the bearings". That is not the case. There is no "interlock". All that needs to happen is that the crank spindle can spin with low friction. 

As you pointed out, the inside of the bearings is smooth and so is the spindle. I guess it's possible that they slip relative to each other, but so what -- the spindle/crank still rotates, the chainring rotates, and power is transferred from the pedals to the crank arms to the chainring to the chain to the rear wheel.

In practice, the friction between the spindle and the inside of the bearing is probably greater than the friction between the inside of the bearing and the outside of the bearing, and therefore the bearing spins.  I assume this is the case because the bearing is designed to have low friction, while the spindle inside the bearing is basically like a shaft inside a bushing (a cylindrical hole), which has more friction than a bearing; otherwise we would not use bearings.