L40s Ultra AE Wet Pads

Does the robot vacuum itself have a liquid reservoir that it uses to wet the mop pads as it’s working ? Or the mop pads are only getting wet from the station?

It depends on what model you have. Some of the budget friends vacuums like the D line will not have an on board water tank, while the higher models like the X series will always have an onboard water tank that continuously wets the mop pads while they are mopping.