How to identify the transistors that are active loads
Can someone explain? Which transistor is an active load for which
transistor?

The main usage of BJTs as active loads is in (for example) class A amplifiers. The circuit you have shown is a class AB design (a standard op-amp design) and they use a push-pull output stage so no active load on the output. However, Wikipedia defines an active load as this: -

Most commonly the active load is the output part of a current
mirror and is represented in an idealized manner as a current
source.

So, on that basis, the active loads are shown below: -

Although Q9 and Q10 (red boxes) are fundamentally parts of their own respective current mirror circuits they are acting as loads to each other and therefore should be considered (as per the wiki definition) as active loads.
You could make an argument that Q13 (blue box) is also an active load to Q17.