Next to your Data, Work and Int class, we now have the UI-Pages class for quite some time. The way I understand it, is that this class is a place to store elements for landing pages. Landing pages are the pages you want to open when you are not opening cases, for example a dashboard, a task list, etc.
In order to show these UI Pages aka Landing Pages in Cosmos you are supposed to add them to your channel's navigation menu in the app studio. There are several OOTB landing pages available already.
I have been looking around App Studio to find a way to add new custom Landing Pages to my Navigation Menu.
Unfortunately, none of the documentation or academy articles describe this in the way that 8.7 is allowing you to do it. Certain buttons are not visible, or options in lists are just not there.
The way I was able to do it was to go into the Developer Studio and create a new Harness Rule in the UI-Pages class of my application. After I created it, I just had to set the rule as a relevant record so it could be used in App Studio.
The new Harness was available immediately in the list of addable Landing Pages in my channel configuration.