https://www.nand2tetris.org/ has been my favorite "computers are not magic" course for a long time. It is a much more hardware oriented program than this post though.
Understanding is one of those things that is very tricky to actually pin down. I don't actually think anyone truly understands how modern computer networks really work. We have abstractions for making sense of computers but every time there is a widespread outage of some critical software/hardware system is an indication of the fact that these systems are now too complicated for anyone to really understand. If people truly understood computers then we wouldn't have so many outages.
