What are the basic CORE functions built into an API like SHOPIFY. RUBY.

Hello everyone.
I have been doing a lot of learning in the past year first with EE and now RoR. ALTHOUGH ITS BEEN A TIME OF ABSORBING TUTORIALS and trials, I have managed to build a few websites on ee. After working out with ruby on rails, am sold and burning to do something with this knowledge.

I have a during desire to know what the CORE FUNCTIONS OR BASIC FRAMEWORK of a web application the likes of basecamp, shopify etc are? What holds everything together that allows an API to be released for further development by the community? At what level is the API placed in the whole Application, and a what level are the Plugins and Extensions?
What is the BASIC ARCHITECTURE of an App like Shopify for example?

Am burning to know this.

Please anybody who can help with their thoughts, thank you so much.

Nelson