In this PnP Webcast, we concentrated on SharePoint Framework Extensions developer preview. SharePoint Framework Extensions provide you capabilities for customizing the user interface of "modern" SharePoint sites and experiences. As part fo the dev preview, we released three different extensions, which are Application Customizer, Field Customizer and ListView Command Set. There are specific Yeoman templates for all of these to get started with the development. Please have also look on the SharePoint Framework Tutorials, which will guide you through the process of building custom extensions.