I was wondering, how you could do something like this, e.g. make the separator line usable, in this example as a display of votes. I think the separator line is a cell here. When you click on it a popover menu comes and you can click what you want to vote for. My question is: Does someone know how to do that or has a link to a useful explanation?

I don’t think that the “seperators” are actual cells, but rather part of the previous cell. You can see that the indicatorview is seemingly not centered vertically (which I personally even find sloppy).
So, just make a custom cell with your desired behavior. Maybe google for “Accordeon cell” or something like that, I think I remember they talked about cells like that in the previous WWDCs.