The single thing to understand: it is islands
Abu Dhabi is not a city centre with suburbs. It is a set of islands with the attractions spread across them, and the map flatters the distances badly.
Abu Dhabi Island holds the Corniche, the old city and most hotels. Saadiyat has the museums. Yas has the theme parks and the F1 circuit. The Grand Mosque sits on its own near the mainland bridges.
Saadiyat to Yas is 20 minutes by road. Corniche to the Grand Mosque is 20 minutes. Yas to the Grand Mosque is closer to 30. None of these are walkable and none of them are on the way to each other.
Plan a day around one island plus one stop, not around a list of sights. That single decision does more for an Abu Dhabi itinerary than any transport choice.
