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A Mosfet inverts the input voltage if the input voltage is high enough to turn on the Mosfet. For an N-channel Mosfet a positive going input at the gate causes an output voltage at the drain to drop.
Many Mosfets have a threshold voltage (voltage from gate to source) of 2V to 4V for it to conduct (drain current) only 0.25mA. It has a transconductance (input voltage change causing output current change) that is low for one Mosfet or fairly high for another Mosfet even if they have the same part number. The average transconductance is high so with plenty of negative feedback you can have a voltage gain of 2.0 if the circuit is designed correctly.