If you open up salesforce you have all these resource types and pages, but it's all fully customizable. There's a built-in programming language with many affordances to work in a dynamic environment (stuff like handling API changes), and you're basically able to point at a thing and get a breakdown of what it's like.

On top of this third party developers can build in stuff that is more or less first class to the system, and it can run on the third party systems but you can get away with using Salesforce's "compute". So you're shipping software for the Salesforce VM, so to speak.

I would recommend playing around with the system to see what's going on.