Extend/Improve UI for specific device (MQTT)


I would like to extend the climate/radiator UI (over MQTT) to use the week plan configuration supported by some devices but I’m a bit lost where to start that.
Are these only home assistant UI changes or also backend changes? Is there some new entity needed for this?
It would be helpful if someone could provide me links to the relevant parts/files used for example for the color selection of bulps.