Yes, defiantly there are lot of .js files in blockeditor directory. but, you do not need to look each and every .js file.
Sorry @abhi0 but I think no one can tell you particular which file or files require updates, you need to figure out by your self if you want to add new .js file then add to source code. We all are interested in output.
I was a part of GSoC-2021 last summer and here are my documents related to GSoC with MIT App Inventor...
Accepted Project Proposal:
Project Summary:
Community Post:
Most of commands are same as you mention in your post, tell me where you got stuck.
The UI of the App Inventor is made using Google Web Toolkit(GWT) and written in java programming language after that some how it will compile by GWT and converted to javascript and the compiled javascript is responsible for UI which you are seeing during development.