The j-fet source is at the bottom of the symbol. The mosfet source is at the top of the symbol.
The j-fet is n-channel and the mosfet is p-channel. The work kind of like an npn and a pnp except their gates are very different, the logic is the same.
The voltage at the bottom of the j-fet must change as you move the dimmer.
The 10 k resistors just center everything for the benefit of the op-amp working in the center of the voltage supply.
Hello #12 measured the voltage change while varying the dimmer pot on my prototype. The voltage at - or pin two of the 1st opamp does not vary much 6.5 to 6.45 volts. I assume Vcc is the battery voltage. I measure 13 volts on the battery. How does one get Vcc -10% at position "I" on this circuit? Thanks Ed