This is the way I understand it. First, there is
MMDVM firmware
that runs on a microcontroller that acts as the digital voice modem. Popular microcontroller for this are the Arduino DUE and the STM32. Second, there is a single board computer that runs the
MMDVM host software
which communicates with the microcontroller that's running the MMDVM firmware. Raspberry Pis are popular single board computers used to run the MMDVM host software. For MMDVM based hotspots, the microcontroller running the MMDVM firmware typically connects to the single board computer via the GPIO (General Purpose Input Out) pins. This is what's referred to as a MMDVM modem HAT (Hardware Attached on Top). Pi-Star is a web based interface to the MMDVM host software. Pi-Star runs on the same single board computer as the MMDVM host software. There is also the MMDVM protocol which allows devices running the protocol to connect.
To boil it down there are two main pieces to a MMDVM hotspot.
1.
MMDVM firmware
running on a microcontroller that provides the digital voice modem
2.
MMDVM host software
running on a single board computer that communicates with the MMDVM firmware (i.e. modem) running on the microcontroller.
Since the openSPOT is not a MMDVM based hotspot, you may want to check out the
Pi-Star forum
for more information on MMDVM.