Quad9 does not make money. Quad9 is a public-benefit not-for-profit foundation. It's supported by donations. Which are very much appreciated right now, because German lawyers aren't free.
Essentially they are the good guys providing a public service
So your answer is.......?
they don't make money, they exist as a public service and AFAIK are funded as a non profit by people of good intention, not from related business
Disclaimer: I know one of the principals, for whom I have a lot of respect