I recently got the chance to work with WidgetKit, and wanted to share my experience of the resources I used for the same.
First and foremost, the documentation is solid this year. "No overview available" is rarely there. So, going through the documentation before starting with WWDC sessions really gave me a gist of what Widgets are, and how they work.
The second resource I really liked was great WWDC sessions. The order in which I followed them -