Nifi registry - extension-repository, extensions, bundles

Hello I'm playing with Nifi and the Nifi registry and trying to understand the features and capabilities. I noticed that in the Nifi registry API there are endpoints for "/bundles", "/extension-repository", "/extensions" but I don't understand how they are used nor intended to be used. I've

Use a different runtime

Hello I want to use Nifi's UI and APIs (i.e. registry) but not use the Nifi scheduler, controller, runtime. I want to prototype hooking in my own scheduler, controller, runtime. How involved would this be? Would it be more involved than replacing the class