How to tell what fraction of current is passing through which BJT
Current thru each transistor depends on load impedance and matching Vbe but it can range form 0 to 4mA with the sum of both not to exceed 4mA.

{ assuming real-world question, change Nfet to Pfet}
Otherwise Nfet is OFF and body diode drops 0.7 not shown to Vout with Q1=4mA Vg1=2V.

Since Vg is limited to between 10V and 2V with an 8V difference , for best regulator the Vgs(th) must be around 1/3 of 8V or 2.7 max so use Vt or Vgs(th) ~ 1V.    Then RdsOn will determine what load you can handle.

Since there is no short circuit protection, you may want to add current sense to limit Vgs drive.

Voltage regulation is not as good as LDO's here due to BJT gain of single stage diff Amp.  and higher Iq due to bias R's

Also since the R ratio is 2:1 the Vout is (2+1)*Vref(2.5)=7.5V out ideally but with bias error closer to 7.7V