Arduino Based Digital Level Code Review - How does it work?

Follow up to the Arduino Digital Level build: Arduino Digital Level using Nano + MP...
Many have asked for a walkthrough of how the code works so here it is!

The most important thing when copying my code from Github is to use the correct matching libraries that I use. If you are not sure how to install them check out this Quick Tip video, it's easy! Easily Add Arduino Custom Libraries F...