The current GCC C SDK. It’s common that a Microcontroller’s C SDK gives full control and access to powerful tooling, but takes a significant amount of local environment setup to make use of. Conversely; Arduino is very easy to set up - because the toolchain has been packaged in a standardised way - but the Arduino platform comes with some designed-in limitations (it’s supposed to be suitable for beginners). With PlatformIO, you get the best of both worlds; a board/chips full, intended development environment, just made available through a standardised ‘PlatformIO’ format that can be used from VSCode, JetBrain’s CLion, Command-line and other tools. I hope I’ve explained that well enough, suffice to say, check out PlatformIO, it’s really making waves in the hobbyist/microcontroller space.