Populate fields with selected item in repeating group

In the sidebar I have a repeating group with items.
In the content section I would like to show detailed info on the selected item from the repeating group.

Is this done using custom states? If so, how? I cannot manage to set a proper custom state through a workflow on a (group in the cell of) the repeating group.
This is what I tried. But then what?
2023-05-03 16_56_33-CoachBPT _ Bubble Editor en nog 7 andere pagina's - Werk - Microsoft​ Edge

Also I would like to highlight the selected item in the sidebar.

What content type is Geselecteerde sessie? It shouldn’t be text, it should be Sessie.
Also, could you provide a screenshot of the sidebar and the area you want the content to be?


Here were a few screenshots of what I was doing.
I found out how to do it using this tutorial.
How to send data from Repeating Group to Popup in Bubble.io (nocodeassistant.com)