That’s because the control circuit is hidden in a monolithic IC. If you’re really curious, here’s a datasheet for an old fashioned design with a block diagram a

That’s because the control circuit is hidden in a monolithic IC. If you’re really curious, here’s a datasheet for an old fashioned design with a block diagram and theory of operation described that should give you some hints. https://www.meditronik.com.pl/doc/plus/zn409.pdf https://www.meditronik.com.pl/doc/plus/zn409.pdf If that sort of thing interests you, there’s a whole field of control theory to study.