ORing mosfet controller with ability to switch off the load
The point here is that the MOSFET is not being used as a switch, but as an "ideal" diode (almost zero forward voltage). This is achieved by operating the MOSFET in reverse mode.

If you simply used a regular diode instead of a reverse-operated MOSFET, you would get an undesired voltage drop and a (potentially high) power dissipation in that diode.

You can avoid that by using a reverse-operated MOSFET as an "ideal" diode, trading-off complexity for performance. But the operation fundamentals of the circuit will be the same: diode ORing of the power supplies.

The key to understand what's happening here is the fact that the conduction channel doesn't have any intrinsic polarity, it acts pretty much like a short circuit (low resistance path) that we can control.

Now recall that the conduction channel is shunted with the body diode of the MOSFET. So we can take advantage of this to build a controller (and TI, Linear, and so many others did) like the LM5050, that monitors the drain and source voltages of the MOSFET and choses when to short circuit the body diode or when to leave it do its "diode role". This way an "ideal" diode can be synthesised because we can get the effective forward voltage as close to zero as needed (or as much as is practical to do it) .