The whole power amp can be thought of as similar to an opamp in terms of how the gain and feedback are configured. There are two inputs, the non inverting input (base of Q302 and 304) and the inverting input (base of Q306 and 308)
The whole circuit follows a rule that can be simplified down to saying that 'the output will do whatever is needed to make the difference in voltage between the two input equal zero'.
If we apply +1 volt to the non inverting input then the output will shoot toward the positive rail. When the feedback loop is connected then when the voltage at the inverting input equals that at the non inverting (and so the difference between the two is zero) then the amplifier reaches a stable state.
If the feedback resistors were 9k and 1k then with 1 volt applied as an input, the output would need to go to +10 volts in order for there to be 1 volt generated at the inverting input So we have a gain of ten. 9k/1k +1 is ten.
If you study basic opamp theory it will all make sense 🙂