“Under Apple’s current App Store policies, all web browsers for iPhones and iPads are required to use the built-in version of WebKit, essentially making all browsers act like Safari.”
Google and Igalia have Blink running on an iPhone 12 in a bare test browser that Apple’s rules forbid from the App Store. Google says it has no plans to ship it. You do not port a browser engine for fun. Google is building the replacement for Safari’s engine now so it is ready the day Apple’s rules loosen.