The display driving code is part of their "xochitl" binary, which is not GPL (and might not be allowed to be, eink-the-company is rather protective of their tech and waveforms). ~All modding happens by reverse engineering the binary to call into that code, either to change behavior (rmhacks) or to act like a display driver for other software: https://github.com/ddvk/remarkable2-framebuffer

The rest has been released a few times, though last I checked it was quite out of date. Far from ideal, but still significantly better than most companies: https://developer.remarkable.com/