Use case is: For RTE toolbar , need to add new plugins. Create a tool bar for TextArea widget.
I am thinking coral ui js needs to be written for all these. But I do not think online coral ui documentation gives any inner details of coral js functions and usage.
Check Granite documentation at Welcome to Granite UI’s documentation! — Granite UI 1.0 documentation
source code you can find in jcr repository, to search individual clientlibs js/css you can use dumplibs
For rich text editor you can check
A good place to start for digging deeper in this is here -- Home | CoralUI