How does this PMOS tube circuit realize bidirectional communication?

There is no problem with the circuit.

What is the capacity of capacitor C1? Is it an energy storage capacitor?

There is a freewheeling protection diode inside the enhanced PMOS tube, so the DS is turned on, but there is a diode voltage drop, and then whether the transistor is turned on or not controls the G pole voltage. When it is low, the SD is turned on. So it can be two-way guide, but the schematic diagram of the original poster is not comprehensive enough, I don’t know what the specific function is.

The R1 resistance measures the charging current, while the FG1 output also supplies power to the system, and then charges the battery through U2. When the system is powered off, the battery current returns to the system. Charging voltage 27V, battery 24V 26AH