Those scales are just medical scales. They used to have them in all the doctors offices...now they're mostly digital.
The way they work is you stand on the pad. There is a rod that goes up to the top part where the weights are. The weights are just counterbalances that you move to equal up the weight that is standing on the pad. When you reach the correct weight the top bar will balance in the middle.
It's kind of the same principle as a pendulum scale or the scales of justice...weight is counter balanced to figure out how much something weights.
The reason they use them in MMA and boxing is they are more accurate and harder to tamper with.