As far as I know they initially essentialy leased out the technology to an independent operator, but stopped doing that quite a while ago. No hard source to point to tough.
It's the same model they use for operating in China.
From an operational perspective this makes things very hard - as you don't actually get access to the services - you need to run an operator from the independent company through the troubleshooting / mitigation for any incident.
This sounds as if Microsoft would not have access to data, which is managed by T-Systems independently. But there might of course be something in the fine print.
My browser is set to prefer English, cookie banners etc. are working, but the rest of the site won't switch, cannot find a button to manually switch either.
[0] https://geschaeftskunden.telekom.de/digitale-loesungen/softw...