Quantum dot LED TVs don’t actually use quantum dot LEDs (yay, marketing). They’re built like any other LCD, but instead of having a white backlight (typically a blue LED with a phosphor to fluoresce the blue to green and red, too, making white) and then a colour filter behind each pixel subelement to only let the right colour through, they have a blue LED backlight, and then a quantum dot film that fluoresces the blue to the right colour.

The advantage of this is that you’re not making light in colours you can’t use just to get absorbed by the filter and turned into heat, so can make the backlight brighter, which, when combined with other techniques to make good LCDs, is enough to make them comparable to OLEDs in quality and price.

Actual quantum dot LEDs let you make light at practically any frequency you want, like OLEDs (traditional LEDs only make light at bandgap frequencies for atoms of elements, and there’s not a huge choice of suitable elements, hence blue LEDs taking decades to materialise after other colours were cheap). In theory, quantum dot LEDs won’t have burn-in problems, but they’re currently not practical to make a TV out of, giving marketing people plenty of time to weasel out of their fuckup with naming existing QLED TVs.