From Shri Khalpada comes a great explainer of GNSS-technology1. From basic building blocks to the full system, it covers many aspects that are essential for GPS2 and Co. to work: travel time, satellite orbits, trilateration, clock correction, incluences of relativistic effects, and more.
It’s a very nicely done explanation of a largely invisible technology that is fundamental to many aspects of life. Every visual in Shri’s article is interactive and many of them invite experimentation.
Shri included a shout-out to a much-more-in-depth explainer of the same topic published by Bartosz Ciechanowski (who seems to specialise in this kind of thing). You have to bring substantially more time for that one.