To study the anatomy of electronic circuits, you need to practice dissection. This is how I see it.
Consider the part left of Q0101. This part can be studied independently of what is happening on the right side. This is a block that can be studied easily.
Surely you have understood the role of D0101; it prevents voltages above 12V or voltages below 12V to be applied at the input. This number depends on the rating of the zener diodes and it may even be +/-15V because the input is labeled at 12V. It serves no other purpose. If the input is +12V<U<-12V, this has no role.
The four capacitors with the inductor forms a basic pi filter. The electrolytic capacitors are in parallel with ceramic capacitors for fast response.
What does the pair R0101 and D0102 doing here?
Under steady state, the voltage across the inductor is zero. Then the diode does not conduct.
If the load (Q0101 on) takes an appreciable current, the D0102 conducts and bypasses partly the filter. So D0102 must be fully rated for the load current.
If the input voltage is >-12V but less than 0, the diode D0102 does not conduct but the inductor must pass the full current. However, Q0101 blocks current.
So the role of the circuit left of Q0101 is:
1. blocks input above 12V or below -12V.
2. Filters noise or acts as a low pass filter.
3. When fast switching or large currents are needed, R0101 and D0102 comes into play but only if the input voltage is >0.6V (diode drop).
4. The pass transistor Q0101 prevents negative voltage.