YetaWF uses TypeScript and SCSS files in all packages. These are always located in a package's ./Addons folder.
Editing these files with Visual Studio 2019 is not recommended. Visual Studio Code is much better suited to edit .ts and .scss files.
When using Visual Studio Code, it is best to open the YetaWF folder ./Website. This provides access to all packages' TypeScript and SCSS files in the folder wwwroot/Addons. All files in the folders in wwwroot/Addons are symlinked to the "real" files in each package's ./Addons folder. By editing files in the ./Website/wwwroot/Addons folders, IntelliSense and all references resolve correctly. This is not the case when editing .ts and .scss files in a package's ./Addons folder.
Visual Studio Code requires the ESLint extension to be installed to take advantage of IntelliSense.