completely clear now.......:)
Roughly speaking you run your computer full-out doing arithmetic calculations for a certain number of days and Bitcoin Central issues you some small fraction of a bitcoin. Bitcoin Central doesn’t have to actually monitor how busy your computer is. Instead you just send them the results of the computation to prove that your computer did the work, and then they issue you the fraction of a bitcoin.
For a given amount of computation, the fraction of a bitcoin that you get keeps getting smaller as time goes on so they don’t use up the limited remaining (virtual) stock of bitcoins. This means it gets more and more expensive in terms of electricity to produce a bitcoin.
My son was mining bitcoins a decade ago when he was a student at UCLA. He was in the graduate dorm with unmetered electricity, so UCLA funded his bitcoin mining. I suspect student dorms don’t have unmetered electricity anymore.