Automatic power switch between USB and LiPo battery
Note:
In the M4 Feather schematic the DMG314 part number appears to be incomplete. Searching at both Mouser and Digikey it seems as a DMG3414 is an N-FET and a DMG3515 is a P-FET. The circuit design would extrapolate to that transistor being a P-FET however if that was the case the symbol of the part is oriented wrong (i.e. source and drain need to be interchanged).
However:
If it a properly connected Pchannel MOS FET it turns on when the gate voltage is less than the source voltage by at least the threshold voltage.
In the M4 Feather circuit the source of the PFET is connected to the battery voltage. When the USB cable is disconnected the gate of the PFET is pulled to GND through the 100K ohm resistor. As long as the battery voltage is higher than the PFET threshold voltage then there is a -VBAT being applied across the PFET drain to source which is plenty to turn on the device.
In a way the M4 Feather circuit is almost just like that second circuit that you showed.