Maybe it has some kind of scale. But it probably mostly handles it by tracking humidity within the chamber. Some of these modes don't even bother showing a time when they start, they'll cook for a bit and potentially power cycle the magnetron for a bit until it then sets a time and finishes. It complains when you open the door.

I've used a number of microwaves with these modes which asked for a weight. Those were pretty untrustworthy on their results, because moisture content and starting temp can vary greatly.

I stumbled upon them, but I strongly think the mid-2000's era GE Profile are probably some of the best appliances ever made. They were at least the best dishwasher, oven, and microwave I've ever used.