I saw a lot of tutorials of how to code a new widget, but I didn't get where the code of the widget suppose to be, or where to upload it.
can someone explain to me step by step how that works?
Hello there,
If you are using WAB developer edition, you can add the new tool in Widgets folder directly. You can check that folder to see the built-in tools files.
Configure Web AppBuilder for ArcGIS Developer Edition (esri.com)