It doesn’t need to make money. A good web browser is a standard part of an operating system these days. Apple can’t ship without one. You might as well ask how they monetise Finder or Notes.
Strategically it makes no sense to not own something that important.
Remember: Safari was created when Apple’s 5-year deal with Microsoft that made Internet Explorer the default browser for MacOS X expired in 2003.
10 years later. Google forked WebKit to create Chrome.
If we're going to split hairs over the whole "Blink is an inferior WebKit fork" brouhaha, we shouldn't forget who Apple sherlocked to get there. After all, turnabout is fair play.
[0] https://blogs.kde.org/2005/04/29/bitter-failure-named-safari...
Similarly, Safari isn't clouds and rainbows either. It serves the same purpose IE did back in the day; furnish a "premium" experience that is deliberately irreplaceable and intertwined with the OS. We saw this with the push notification API, "Add to Homescreen" functionality and so many other places where Apple dragged their feet and refused a featureset that would enable competition with native apps. This is a hell of their own making, Apple can leave any time they want by acquiescing to app publishers the same way they did on Mac.