Cross coupled Class AB output stage
Because M1 is in source follower configuration (as is M2). So M1 source follows Vin. LH circuit works as follows : Assuming Vb2 constant, M3 is a common gate amplifier, so its drain voltage (and M6 drain, M6,M7 gate voltages) also increase.

M3 drain is a high impedance input so M1 source voltage can increase all it likes with virtually no change in M3 drain (and source) current. Therefore the small signal gain is approximately 0.