Some Arduino Reference Materials
In addition to the resources posted at arduino.cc a quick google search reveals thousands of references (of varying quality and usefulness) I've started digging through these that I found more useful than most:
https://www.hackerearth.com/blog/developers/arduino-programming-for-beginners/
This one is a huge reference manual about 900 pages long (but some pages at the end seem to be copy/paste errors). It gives enough hardware background to understand the programming as well as being an Arduino programmer's reference. It looks like it was created for a college lab guide:
https://www.ele.uri.edu/courses/ele205/ELE205Lab/ELE205_Lab_files/Arduino%20-%20Reference.pdf (3.7 MB PDF)
Here is a guide to install the Python programming language instead of the default C++ tools. Keywords to search on would be MicroPython and CircuitPython:
https://cdn-learn.adafruit.com/downloads/pdf/welcome-to-circuitpython.pdf