It's a fair question. Energy charts has both the physical flows and trading charts for Germany.

E.g. trading https://www.energy-charts.info/charts/energy/chart.htm?l=en&...

Physical flows https://www.energy-charts.info/charts/energy/chart.htm?l=en&...

And it's not the same, so sometimes there is just "transit", although transit here should I guess be understood very loosely. I also assume they just know who paid for what.

Edit: although upon re-reading I think you probably asked something else. Sorry in that case.

Disclaimer: I'm working for an actor in the electricity market, although not on the business side so I don't know everything about it.

They do know who paid for what. And actually anyone is able to know. It's a very regulated market, and most of its data is published, although I have to admit that if you don't know how to fetch it and read it, it's a bit hard to figure out by yourself.

You should be able to find published data on the ACER Remit Portal (https://www.acer-remit.eu/portal/home), ENTSO-E Transparency Platform (https://m-transparency.entsoe.eu/) and JAO website, Market Data tab (https://www.jao.eu).