With the ZMK firmware they're using the sides usually communicate with each other. You have a "primary" side which does most of the firmware heavy lifting, including the Bluetooth connection to the PC, and a "peripheral" side which just sends raw key codes to the primary.

That arrangement means the peripheral side gets significantly better battery life than the primary, so ZMK also has an optional "dongle" mode where you connect a third ZMK device directly to the PC over USB, which acts as the primary, and then both sides of the keyboard act as peripherals so they both sip power.

As a bonus the dongle mode also means the PC sees it as a wired keyboard so it works in the BIOS, etc.