How to claim free disk space in APFS after deletion of huge amount of files? Given: Macbook pro 13 Inch with MacOS 10.15.1 (Catalina), 500 GB SSD built in.
Hard drive had 34 GB of free disk space according to diskutil and df -h.

Wanted: Reclaim  free disk space.

What I did:

I moved a huge folder of 120 GB onto an external disk. Trash was empty. I rebooted the machine, ran Disk Utility's First Aid as well.

It remains with 34 GB of free disk space.

I moved another folder of 9 GB to an external disk drive.
It still remained with 34 GB of free disk space.

I gave the system some hours of probably internal reorganizational tasks (if any... because you'll never know)

Then I figured out 2 further cache folders on a project holding 25 GB each and removed them via rm -rf. 
du -sh gave me proof that these 50 GB was really deleted and my whole projects folder was 50 GB smaller than before. 

However, my disk is still quite full. Free space: 34 GB.

How can I fix this without reformatting and reinstalling everything from scratch?
Okay, I can answer by myself.

In further investigation I came across the hidden space called local snapshot space that is caused by local time machine snapshots and tried to purge them for giving back at least 100 GB:

♠ sudo tmutil thinlocalsnapshots / 100g 1 
Password:
Thinned local snapshots:
com.apple.TimeMachine.2019-12-07-134712.local
com.apple.TimeMachine.2019-12-07-134712.local


Although some snapshots should have been deleted, nothing happened in order to show more free disk space. I increased the argument to 110g and now at least 72 GB was released and show up as free disk space.
I plaid a bit with separate values, found a tool called Daisy Disk that is able to show the size of hidden spaces.

I tried to remove all these local snapshots with 

tmutil deletelocalsnapshots /


and it seems to remove them. At the end there are still 50 GB of hidden spaces left according to Daisy Disk that I am unable to find their location at the moment.

It seems that under the hood APFS would care by itself to free up this space if required. However, the UX around all this is as bad as possible and a huge black box from the perspective of a user.