I have never seen the search contract so I don't think I am breaking any kind of NDA but I have been close enough to engineering and browser planning and product management to know that Google dictates (negotiates?) in great detail every aspect of search in the browser. This is not a simple "you can use Google Search" deal, instead Google is basically the product manager and ux designer for the browser and they spell out in the contract how search works, how the navigation works, what the UX looks like, what is allowed and not allowed in terms of competing search engines. On mobile it goes further than that and they also dictate how the browser integrates with Android features related to search and voice search.
Mozilla can't surive without Google cash and I fully believe they negotiated hard for a good fair deal that puts users and choice first but I am also sure that Google got the better hand and that is why Search in Firefox has been largely "product managed" / micro-managed by Google.
Apple probably had much more freedom becsuse of their size and power and I don't really understand why it is not possible to add a custom search engine. There is no advantage for Apple to not allow this. Other than .. Google probably force a good deal in their advantage? Apple taking more money by allowing less choice, or even being forced to accept a new deal?
Not having a search deal is simply not an option. Mozilla would die. Apple would have to explain billions of revenue loss to shareholders. Both are impossible.