The answer depends on the level of the detail you are asking for. There is a generic explanation, slightly more engineering / scientific one, relying on basic e

The answer depends on the level of the detail you are asking for. There is a generic explanation, slightly more engineering / scientific one, relying on basic electrical engineering knowledge with not much nuance one and finally full detailed engineering explanation with a whole bunch of calculus and nuances one. CRTs are yet another circuit component. They rely on electrons moving or getting stored in them to "work". If you increase the voltage, the electrons will move faster through the gun and crash into the phosphor coating with more kinetic energy. Or one can increase the current i.e. the number of electrons hitting the CRT per unit of time. Both results in a brighter spot. Both requires one to increase voltage of some circuit at some point of the upstream. Since the TV signals are also an observable piece of voltage information, which we receive via an antenna, we can create circuits that are capable of detecting that voltage and simply act on its strength. We can create a circuit that acts when a higher voltage level is observed and cause the dot to be brighter. We can create a circuit that has repetitive behvaior like changing the magnetic field in the deflection coils such that it periodically moves left to right. So it will draw a line. We can also create circuits that act on time-dependent behavior like the voltage dipping down for a tiny fraction of a second to a level that cannot produce a dot. So we can detect when we should move the deflection coil's magnetic field slightly downwards to draw the next line. Or even bigger dip to reset the position all the way to top. When you combine all those circuits into one big Rube-Goldberg machine and adjust the thousands of literal and figurative knobs, you'll get a TV.