Microsoft are great at documenting the changes to Windows 10 Insider builds via the Insider blog posts but they never document any changes to Windows Mixed Reality. There are plenty of changes to the VR platform, which will it probably share with HoloLens 2.
The latest change I have noticed is that in immersive apps you can now launch Windows apps without closing the running app. Currently if you in an immersive app or game and you hit the Windows button on the motion controller you are presented with a mini-menu. One of the options being to go back to the Cliff House where you could then launch other apps.
In 19H1 when you hit the Windows key it now brings up the Windows Mixed Reality Start Menu when you can launch apps without going back to the Cliff House. This means you can read your email or check twitter without leaving the game or app you are currently in.
Another change is the new pickup icon where you can move and re-size windows.
Overall Windows Mixed Reality is looking great in Windows 10 19H1. Once 19H1 hits the slow ring I will record a video highlighting the changes.