An example app can be seen on the demo-final branch.
master branch contains starter code repo for creating ESRI style apps with the following libraries:
1. ApplicationBase (https://github.com/Esri/application-base-js)
2. ArcGIS 4.x JS API w/ESM (https://developers.arcgis.com/javascript/latest/es-modules)
3. ESRI Design System (https://github.com/Esri/calcite-components)