My colleague, Nick Black, and I gave a talk to a room full of GIS developers at Esri's 2018 Developer Summit conference. The goal was to give them some insight into our design process on the Apps team (design methods, how we collaborate, tools used, etc). These points were reinforced throughout the talk with examples of the ongoing redesign of Collector for ArcGIS.
And finally, in an effort to make some of this work easily accessible and more visible, we hosted our components on Storybook. Here, people can interact with the actual, final versions of the components.