Due to the nature of my job, I interact with some of the more noteworthy geospatial experts on a weekly basis. These people will take any time of the day to explain to you all of the microscopic nuances about how any GNSS works.
The basic operation of a GNSS with ~30m accuracy is literally explained on one slide in lecture 1.2, and from then on the rest of the course is about implementation specifics, and enhancing performance.
The basic concept at its core is so elegant and beautiful, but no less interesting are all the ways this system is augmented with different techniques to improve the accuracy down to ~1cm.
This is one of those resources I've downloaded and made sure to have backed up, because it's so invaluable to me.