Here's an awesome quaintly retro short YouTube video that does a great job animating what exactly is going on in the CRT tube. Though Kavyon's explanation was clear and concise, I think this video does a better job at illustrating how the CRT might work from a system level. A cool story that builds off of this CRT technology is that of the Sony Trinitron CRT. It was very popular when CRTs were still a thing (I think my family might still have one stuffed away somewhere) Here's an article that briefly talks of the innovations of Sony and how they revolutionized the CRT.

For whatever reason my links did not show up, so here they are again: https://www.youtube.com/watch?v=Gnl1vuwjHto https://spectrum.ieee.org/consumer-electronics/gadgets/the-consumer-electronics-hall-of-fame-sony-trinitron

How does a single electron beam create an image containing anything more than a single dot? Even if there is one beam each for the RGB colors, it still seems difficult to imagine.

The beam is moving very quickly and sweeping across the phosphor screen. When you watch the CRT with the naked eye, you cannot perceive this sweeping. This slow motion video might illustrate what exactly is going on. https://www.youtube.com/watch?v=8sThyWQC4RY