the SSP and the Engine/Remote shipped with System8 share the same hardware platform which is linux based.
this means you can run linux software on it, as long as it’s compiled for ARM. We use debian stretch as distribution for all development.
So you could run PD on it or supercollider. The Audio I/O is ALSA compatible. The instructions on how to use the SSP or Engine/Remote for development are in the forum.
Our software, which we ship with the SSP and Engine/Remote, is a full modular synthesizer in itself. We don’t use PD or supercollider, the software was developed from the ground up over a period of more than 10 years. You can also write VST plugins and compile them for linux/ARM and load them in our software and use them in the patching grid.