All the 1985s in the Back to the Future trilogy are at the same point in time dimension 1 (1985, duh), but different points in time dimension 2.
The Delorean takes a shortcut through time dimension 3 (think the typical wormhole analogy of poking a hole through a folded piece of paper) to get between the various 1985s and the various 1955s and 2015s, and 1855 (it's obviously not using time dimension 1, since it "vanishes" at one point and reappears at another, or 2, since that wouldn't allow it to get to a different year).
(Of course this is utter nonsense because that's not how time travel works in Back to the Future, but it works as an analogy and it's less complicated than Avengers: Endgame).