A quick googling says Firefox makes most of its money by making Google the default search. While I'm not suggesting they are doing the same, it does make me feel they are aligned as a business.
Apple is the only tech company I know that's primarily funded by customers paying for a product directly. That's why I use Safari.
Another commenter already said it, but given that Google pays Apple $20 billion annually for the default search engine, it's accurate to say that Safari's development is entirely funded by Google.
That Apple has alternative sources of revenue are irrelevant for Safari's present and future. Just to give an example — both Firefox and Safari block third-party cookies, which has an impact on Google's tracking, but have you noticed that neither of them block ads by default? They couldn't even if they wanted to, given it has a direct impact on their cash cow. And Safari's ad-blocking capabilities is what inspired Chrome's limitations from Manifest v3, the latter being more capable, actually.
The more important question is: What happens to Safari if Google's payouts to Apple stop? And there's no good answer.
Apple is the only tech company I know that's primarily funded by customers paying for a product directly. That's why I use Safari.