A Microsoft file hosting and synchronization service.
Okay, so I have spoken with Apple Mac support. Luckily, the people I spoke to were very helpful.
They determined that the on demand OneDrive files on my Macbook take up no disk space. While the storage management application thinks the files do take up the space they would if I were to download them all.
After which they escalated the issue to Apple engineers.
For this, they asked me to supply them with certain screenshots with proof of the issue and a dmg-file with captured disk data (what happens to the disk when I have a file on demand, download it and then clear it from my hard drive again).
I have also shared my entire mail-history with OneDrive support with them and supplied them with the link to this community-page.
A support professional will call me when there is a response from the engineering team.
Fingers crossed.