is made on which is installed the SDcard, but also the 2 voltage regulators to power the MCU and the SDcard. One regulator change the 6v input (5 cells 1.2v) to 5v. The second regulator does the 5v to 3v conversion. To adapt voltage from the procsessor to the SDcard, a set of resistors are used, commonly called "voltage dividers". See the schematic for more details about the resistors and values to use.
connect the PC to the radio, a small connector is made. A USB/serial/ttl cable can be plugged directly into it for download and configuration.