Arcgis REST api is the lowest api, is the foundation of all ESRI web based application.
Arcgis Javascript api is build on top of REST api
Experience buider is high level api build on top of arcgis js api.
You have 3 options to build web based map
Option 1:
use Arcgis REST api only
https://transparentgov.net/json2tree/esri/server/folder.html?org=https%3A%2F%2Fgisportal.hrsa.gov%2Fserver%2Frest%2Fservices&url=https%3A%2F%2Fgisportal.hrsa.gov%2Fserver%2Frest%2Fservices&select_folder=36&select_layer=0&arcgis_online_token=
All web map fall in these rectangle are made of using Arcgis REST api only, NO arcgis javascript api used.
It is like you use lowest level windows api written in C++, you can do max you can do, everything you can do. No restriction, no limitation.
Option 2:
use Arcgis javascript api only ( I occasionly use Arcgis REST api for those function that is not available in Arcgis javascript api )
all web apps in blue rectangle is written in Arcgis javascript api
Option 3:
use high level api only
You could use high level api experience builder, see examplehttps://geocarenavigator.hrsa.gov/?page=Home
or use other high level api based product, see example
https://maps.usgs.gov/padusdataexplorer/