Current flow in trailing edge dimmer
Both turn on when the gate voltage is high, one is shunting the body diode so neither body diode conducts (therefore the drop is proportional to Rds(on)*2). When blocking, one body diode cannot block so almost all the voltage appears across the other MOSFET.

The circuit driving the gates does not see much voltage relative to the “ground” at the junction, the trick here is to make a power supply referenced to that point and isolate signals to that circuit.

MOSFET SSRs could be used if they were fast enough, but they are typically very lethargic in comparison to the speed required- because they have to work with a small photovoltaic current to drive the relatively large gate charge.