Wow, that is really, really good. The tracking is just so smooth!
My guess on combining the astro + ground shots is that he probably makes 2 exposures at each timestamp? One exposed for ground, one exposed for sky, and then HDR each pair of images, and then timelapse the merged images? For some though you could tell that there was artificial lighting for the foreground, so they could have been just single shots.
As for the "sun". I was perplexed at this too (it doesn't seem physically possible) but in the comments he says that it was the moon, not the sun, in those shots. And in one in particular (at 2:10) it was only a crescent moon (maybe 15%

so there was not enough light to blot out the stars. Now THAT makes sense! It could also help illuminate (no pun intended) some of the other shots where the ground seems lit up. With the right amount of moonlight and the right exposure settings, you could get the ground and the stars to both be properly exposed in the same shot. You just have to have your stuff worked out and know the phases of the moon, the weather, etc. and just time it perfectly.