This is the C/C++ SDK documentation. Code for the Pico (RP2040) on https://github.com/raspberrypi/pico-sdk. The RP2040 is supported by the TinyUSB library. There’s a fun script for setting up the SDK: pico_setup.sh.
This is unreal: Using RP2040 PIO to drive a poorly-designed display. I read the code and it made me feel like I don’t actually know how to program. :P