What is the purpose of a power MOSFET in this voltage-peak detector?

Question

I am trying to understand the use of the 3 different power MOSFETS described in this peak-detect circuit. The circuit was designed to measure the voltage peaks from a photomultiplier and the Arduino data-logs and displays the data.
The 3 main circuits from top to bottom are 1. Peak-hold, 2. pulse manipulation, 3. monoflop. My questions concern mainly the peak-hold circuit using the MOSFETs:
Question 1: Since this circuit is low voltage/low current, are power MOSFETs necessary? Why not use transistors NPN transistors or something similar?
Question 2: is the specific choice of IRL3803, IRF4905 and IRL510 important in this circuit? Could these components be swapped with different MOSFETS and achieve the same functionality?
I'm mainly concerned with the use of both IRL and IRF components, since they require 5V and 12V respectively. Is it reasonable to have all IRL components in this circuit?

Marko Buršič · Answer

The MOSFET is used as reset circuit. Why BJT isn't used? Because it would interfere with the sample/reset circuit, as the MOSFET has a insulated gate and low Rdson when triggered - empty the capacitor aka. reset/clear.
However you do need just a very small MOSFET for that purpose.
EDIT:
The bottom circuit seems to be done by someone who has skills in electronics and he knew what he was doing, the upper circuit is made from someone who doesn't really know what he was doing, me neither. Maybe it is useless, so you should rethink on what you need from this circuit.
EDIT 2:
In these days, IMO there are such opamp that could amplify and remove bias even with a single supply.

If this holds true, then:

simulate this circuit &ndash; Schematic created using CircuitLab

simulate this circuit

What is the purpose of a power MOSFET in this voltage-peak detector?

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