Not really confusion, but reading out Ashley's blog posts about Construct 2's architecture, Construct 2 vs JavaScript, How Construct2 helps make HTML5 work (old, there's even been more improvement since), how to write low garbage real time JavasSript and a new way to learn how to program can help you understand even more about how C2 works under the hood.
There's even this article about spritesheet even if it is not directly related to JS.
As far as the SDK goes, the way plugins and behaviors are designed as modules is pretty straightforward and acts as good guidelines.
The available SDK template helps a lot.
As far as I'm concerned, there wasn't this template available yet when I started coding plugins and to "help myself" I used to open and read the already provided plugins and behaviors.
This was maybe the most confusing at the time and still allowed me to get into JS and plugin making pretty easily.
If you're already familiar with JS, it should be pretty straight forward.
One last thing though, I tend to believe you can do almost everything with the events system as it is (I hardly use third-part plugins nowadays) so the SDK is not really required, but it's always nice to know it's available in need.