Reverse-engineering a brushless ESC, could someone help me understand the circuit
The LOW signal can be a logic level signal from the 5V MCU because the lower N-Channel FET in the bridge has its Gate-Source voltage referenced to the GND. Due to this the FET can be turned on with a signal that rises above GND.

The HIGH signal needs to be buffered through the 2N3094 because the upper N-Channel FET in the bridge has its Gate-Source voltage referenced to the output. As such the gate of the FET must be at a turn-on threshold above the output voltage in order to turn on. In fact to get the output to go all the way toward VIN it is necessary for the upper FET gate to be driven above VIN to get the FET to turn on. 

The C6 and D1 components perform the function of a bootstrap circuit of sorts. As the upper N-FET turns on its SOURCE pin starts to rise in voltage which in turn helps to push the GATE of the FET up higher via C6. In this instance the D1 diode reverse biases to allow the GATE voltage to be pushed up above the VIN level. There are better bootstrap circuit configurations that would work a lot better than this one.