Understanding constant current source in amplifier stage
Q5 and its associated components form a constant current source and therefore the collector current of Q5 is held at a constant current which nominally doesn't vary.
The signal at the base of Q7 is a varying current signal, the voltage at the base of Q7 is held pretty much constant at 1.4V.
As the injected base current of Q7 varies, its collector current will also vary. If Q7/Q9s' collector currents increase then there is less current available from Q5 to drive the base of Q23 and therefore the amplifier's output will fall. (Remember there is a constant current out of Q5). If the base current of Q7 reduces, its collector current will  also reduce. This will force more of Q5's constant current output into the base of Q23 and also less current will be pulled from the base of Q25. This results in the amplifier's output rising.