This is for advanced software developers only: MIT App Inventor Sources. That’s the actual source code that makes App Inventor work behind the scenes – from the browser based editor to the server side support to the AI Companion. It’s all there.
With access to the source code it is possible to host App Inventor on your own server and it is possible to develop your own App Inventor components (this also requires making a custom version of AI Companion).
Again, this is for advanced software developers.