current mirror as active load in common emitter amplifier
That difference in the currents between IC1 and IC2 goes into the load attached to Out.

Also the output impedance of a real transistor is not infinite (could be megohms) that will put a limit on the achievable gain.

That circuit is not practical without some DC feedback to set the DC voltage at the collector of Q1 and Q2, as drawn the voltage would either saturate Q1 or Q2.