Best dumbed down version I can give you, since your an emulator addict...and its still not dumbed down enough
You know when an old ROM runs really fast or you set it all wrong and you can see the actually "Lines" going top to bottom?
Thats a scanline...again usually you can't see it and you may never see it again ever since we use HD everything now, and we don't use the "bube tube".
SEGA and SNES relied on the actual old televisions technology (and would no longer work) by using those scan lines (reality at the time is they were normal when they were invisible to the human eye), the emulators drawing the game used the original code and therefore ended up drawing in the same fashion on a better new technology, thats why you actually " see" the scanlines when it doesn't function right.
They both relied on those scans (from the TV cable wire to the system) to determine the position of where the gun was aimed.
NES actually did something like a modern transparent graphic by using the "mask" (pure white or black picture) between graphics (like Mario when you go through the blocks and run behind the wall heh), it simply acted as a trigger to cause the graphic to flash and detected the change in "light" without need to rely on the TV.
The light reflected back at the sensor on the GUN from all the white targets mid-flash and that is how it determines the shot, much more accurate, no sensor or other tech needed, you could shoot directly at the screen (try that on SNES or SEGA...fail lol) and in theory it would STILL work if redesigned for new televisions, where the others couldn't unless they were redesigned all over again.
Far superior in every single way, unfortunately I guess it wasn't cost effective as well.
When people make light guns not based on BT or RF, its generally the same basic concept that makes them work, I obviously don't have much more of the technically details (and its been awhile since I did this reading) or I'd have built one for myself already
Hope that somewhat helps...but its gonna be greek unless I made a live video with an NES and other consoles going in slow motion to let you see the changes on the screen or you understood the tech itself already heh.