The reason for my post was anxiety! I got a new computer that I restored from my old computer with time machine. My old computer had around 50Gb free, which was also the case on the new computer after time-machine restore. But then the free space started to decrease, reaching below 20Gb after updating to 11.4. Like a time-bomb: I was watching it ticking down to 'out of space.' I got really worried. But now it has reversed, free space is slowly increasing, now nearly 60Gb.
Lesson: after restoring to a new computer, the Mac is doing a lot of things in the background which uses space, that will eventually come back. Slightly scary.