I have a blog (messy but lots of info) /blogs/myAlaskaGIS/2016/03/09/web-appbuilder-developer-edition-customization-resource-list?sr=search... that will help you find a lot of links on current custom widgets, getting started, learning resources, etc.
There is an excellent step by step document that might be of help /blogs/wab/2015/05/01/installing-robert-s-enhanced-search-widget-for-the-web-appbuilder it may have a few things out of data for the latest version, but it is much more than just the enhanced search widget. good resource.
But also visit the other site on geonet
Web AppBuilder Custom Widgets Web AppBuilder for ArcGIS ArcGIS API for JavaScript Web AppBuilder Custom Themes and a handful of others that you will find once you start looking around.
re: training. As you mentioned, lots of free training and videos you can watch
http://training.esri.com
http://videos.esri.com
and the Esri Training site has a lot of "class resource" lists for many of their instructor led classes. very handy.
Web Appbuilder (thru ArcGIs Online, Portal or developer edition) has quite a bit of functionality that can be customized without writing any code. Most of the custom widgets have directions for installing and customizing. And if you do need to create you own, the help has a workflow on how to do it, but I would check the built-in and the widgets others have built first, before possibly re-writing anything. Also, ask questions (after a search for it being previously asked) in the Custom Widgets page I lists above. It's a pretty active community.