How do these Zener diodes limit the maximum voltage seen by other circuit components?
D2 and D3 will limit the voltage, as they have limiting resistors, full load current does not go through them.
If there is 60V at Vin, and you assume the zeners to be ideal, D3 will limit to 6.8V and since current is provided only via resistor R3, there will be 60V-6.8V = 53.2V over R3, so only about 0.5mA will flow. So D3 protects the Vgs of Q2 and limits it to 6.8V.
Same thing with D2 really. D2 does nothing, unless voltage over R5 grows to 6.8V, and then it limits the voltage over R5 to 6.8V. This is the Vgs of Q1 it is protecting. And the current is again limited by R6, so only about 2.7mA will flow.