They measure a series of points to get a bed mesh, the same as other leveling sensors do. You shouldn't need to live-adjust z, because you've measured the bed surface with the nozzle tip.

With a BLTouch, you need to adjust the z-offset, because the nozzle tip and the BLTouch trigger height are not the same. Once that is calibrated, it should continue to work fine as long as the sensor is working properly.

I think it mainly comes down to the accuracy of the sensor. If your sensor gives you a +-0.03mm reading when probing the same spot, then you will have high and low spots in your bed mesh that you can't correct with z-offset.

This could explain your issues with the BLTouch you mentioned in another thread here. I have various brands of knockoff BLTouch sensors, and the worst one had a +-0.02mm range when measuring points, which made it impossible to get a good bed mesh. The one I replaced it with had about a +-0.005mm accuracy, which fixed my issues with the low and high spots.