It's tough to get truly "bare metal" code running on their chips once you want to dig more deeply into how things work. Everything is based around their development framework, which is open-source and quite easy to use, but somewhat difficult to read.

Part of why people like the Arduino ecosystem is that it supports a wide variety of platforms, many of which have excellent documentation and register-level example code.

But at the end of the day, the best way to learn about microcontrollers is to grab the first one you can find and treat it like a toy. Have fun!