So far, these are my personal observations of the PinePhone Keyboard. The name of that will change -- oops.

This Keyboard Case is a case for the PinePhone and PinePhone Pro. As the name implies, it adds a keyboard to the phones. It also has a 6000 mAh battery that essentially tripples the phone's 3000 mAh battery life.

Connectivity

The Keyboard Case has six pins that connect it to the PinePhone or PinePhone Pro. They are labeled the same as on the PinePhone and PinePhone Pro. See PinePhone#Pogo_pins for information on the pogo pins.

INT  SDA  SCL
VBUS VBAT GND

The SDA and SCL pins are connected to the PinePhone or PinePhone Pro's I2C bus. This is how the phone and keyboard communincate.
VBUS connects to the USB +5V line. See Charging Information for more information
The VBAT pin connects to the battery. This powers the keyboard if it is out of power.
INT is a pin that's used for interrupts. In the case of the preinstalled firmware, it's used so the phone can request key presses from the keyboard.

Charging Information

In most schematics, including the PinePhone and PinePhone Pro, the +5V coming from the USB connector is named VBUS. This voltage is fed into the System-On-a-Chip in the PinePhone (Allwinner A64) or PinePhone Pro (RK3399S). The SOC then manages this voltage and charges the battery.

The VBUS rail is also accessed through one of the pogo pins on the back of the phone. This is what the keyboard uses to charge the phone. The output of it's IP5209 chip is connected to the VBUS rail through this pogo pin, which charges the phone. This makes the keyboard act as a USB charger with only +5V and GND connected.

Problems with Charging Implimentation

This could change in the near future with a new revision to the PinePhone.

In the case of the original PinePhone, this can be fixed by Pine64 by having the VBUS pin on the keyboard instead connect to the Allwinner A64's second power input, similar to how it's handled on the PineTab. However, the RK3399S used on the PinePhone Pro does not have this feature, which means it may not be as easily fixable on the PinePhone Pro.