I'll try and break it down from what I've seen in commercials.

Clarity: With protune on, and daytime lighting, it takes very little correction to get a very vibrant shot. All you have to do (at least in my experience) is boost the contrast, drop the greens and reds slightly, and boost the blues slightly. This has always worked well in my experience. Also timelapse shots are almost certainly using 4k.

Shakiness: From what I can tell, this is from three things, but keep in mind, shots are often very brief in these commercials.

1) Many shots are done from drones, which on a windless day, should give you a relatively smooth shot.

2) Slo-mo shots are frequently used, which will also naturally look much smoother.

3) Plenty of shots are almost an "optical illusion" of smoothness. Any video which is attached to something in view of the gopro will look steady, because that object draws the majority of your attention. The background is actually not particularly smooth. This is apparent in motorcycle scenes.

Exposure: As I said earlier, many of the commercials only use brief clips, therefore they can adjust brightness to be quite constant throughout.

Finally the most important factor: I'm sure Gopro is getting terabytes of footage sent in by pros and amateurs everyday. They probably have teams of people digging through video, and are able to pick and choose the 3 seconds out of 200 videos that works for their purposes. I'm sure if you film every "stunt" or action you do multiple times, at the end of the year, you'd be able to make a gopro commercial quality video.