Measuring high voltage with low power op amp
The key is the Vcc_H and Vcc_L suppl that the op amp is using. It's referenced to the 150V supply, not ground.

And then this OP_A whose power supply voltages are "hanging under" the 150V rail, gets fed the voltages across the current sense resistor. Since these current sense voltages are tiny, they are also close to 150V so nothing fries.

Then the op-amp drives the MOSFET.

You know how a pull-down transistor with a pull-up resistor can be used to convert a low voltage signal to a high high voltage signal (with an logic inversion)? This is similar to that, but upside down: a pull-up transistor with a pull-down resistor lets a very high voltage output a very low one (that is referenced to ground in this case). And instead of the transistor being fully conducting or blocking for a digital signal, it's partially on for an analog signal.

That dissipates a heat, potentially a lot of heat at high voltage differentials, and its heat that the MOSFET dissipates rather than the transistor (you can think of the MOSFET being driven at partially between full on and off as a variable resistor, though a nonlinear one, but the op-amp feedback compensates for that. That's why R3 and R4 are so large, to minimize the current flowing which minimizes heating.