The argument is that Humanity's past knowledge and labour is a common heritage of everyone. Anybody that benefits from it must, at least in part, pay back "the commons" for that benefit.

So you get to leech off the greatest minds in the present while they are living and again after they are dead?

So when people build off humanity's past knowledge and they pay for the privilege I assume the new knowledge that is created does not belong to humanity any longer and belongs to individuals?

I don't get what you mean. How do you mean "leech"? The point is that anyone — actually let's take a concrete example, Google and Deepmind — when they produce something, that was due in part to their own labour and in part to the heritage that the past generations and the current fellow humans have given them. Therefore, in principle, some of the fruits of their success belong to them and some belong to society. It's now a matter to discuss the split ;)