While this is a code along session, at the time of my viewing there was no link provided, I did search the forums and the link here was provided, but it gave me a page not found. Hopefully by the time others see this blog post the link will work.
Check out the Meet watchOS 10 a Design and Build apps for WatchOS 10 sessions.
An initial compile of a WatchOS 9 app against watchOS 10, you will get many features automatically, if you were using standard controls and options. Shifting to NavigationSplitView can be achieved by switching from NavigationStack, moving your contents into the detail View builder, while adding a list view for navigation.
Breaking up long views to Vertical Tab Views make it easier to break up content in to logical aspects.
Adding in ToolBar and setting materials to provide additional information, makes it easy to help users.