how the mosfet work on the circuit?

 

Analyze the circuit the best you can and far as you can?

It might help to get rid of any components not needed for the analysis.

The low hanging fruits are anything marked optional and any bypass capacitors (which, in this case, is all of them).

The just walk through the circuit form input to output, assuming that the opamps do what opamps are supposed to do, which is output whatever voltage is needed to bring the input differential voltage to zero.

 

Look at the first OpAmp and Mosfet.

You know from OpAmp theory if we have negative feedback then
the V- input will match the V+ input, with a tiny error. If V- is same as
V+ what can we say about the current going thru R1, and hence M2 ?

Once you understand that then you know what the behavior is of that
that portion of circuit, eg. whats its purpose ?

Regards, Dana.