If you see jimu, that means it made of experience builder
Jimu is the name of the framework for ArcGIS Experience Builder
For example, open widget folder, there are 2 custom widget
UDS Data table widget
UDS Mapper V2 widget
All the other folder and file are downloaded from ESRI.
Your custom source code only in 2 widget.js file
So you only need to work on these 2 widget.js file.
Also you can tell this experience builder use arcgis javascript api v4.27
https://geocarenavigator.hrsa.gov/?page=Home
You actually will not directly use arcgis javascript api v4.27, instead, you will write your code to use experience builder api.
While experience builder api actually calling lower level arcgis js api.
Experience builder api is high level api, it is build on top of low level arcgis js api.
Again you will work on high level api only, you will not directly work on low level api.
It has good and concern.
Good is fast developement, you only need write your custom code in 2 widget.js file, the rest of source code is already ready in downloaded package.
Concern is lack of flexbility, lots of function can't do in high level api, but can do in low level api.
Same idea in windows application.
C# is high level api, C++ is low level api.
C# is microsoft product, which calling low level windows api which is written in c++.
C++ is NOT windows product, c++ is calling lower level machine language.
You can use C# to quickly develope a app because high level api already take care of the rest of code for you.
However, lots of commercial software still written in c++ directly calling windows low level api to max the function capability.
A lots of things high level api c# can not do, while low level c++ window api can do.
Window's itself is written in low level c++
Experience builder is high level api, like c#, allow you quickly create a windows app or web map.
windows api in c++ is low level api, like arcgis js api, is fundation allow you to do lots of things that high level api can NOT do.
Even high level api itself is build on low level api.