opensource can’t be monetized for advertisements.

It can!

That’s what Google did with Android: Google literally made a free open-source operating system everybody could copy, use and develop for for free, to create a wildly successful ecosystem.

Once the ecosystem was fully developed, slowly, year after year, Google moved features out of the open-source AOSP project and into their proprietary stack. Look at AOSP now: it’s a shadow of its former self.

And now they’re killing off AOSP and AOSP-derivatives and turning Android into their own Apple-style walled garden.

I’d say Google very successfully used open-source to its advantage. Google sure knows how to play the long game.