So - in simple terms - how does this work?
You know all the Batman films that were before this recent Affleck one? Imagine they all exist, but theyre all in their own separate universes, so like Adam West's Batman is Earth 1, Burton's Batman is on Earth 2, Nolans is Earth 3 etc etc.
You dont have to worry about making sure all the individual films fit together, so you could have a Superman film that is in a different world to the upcoming Batman film, they dont have to share a style or even be the same period in history, you could have a Superman in the 70's while Batman is around today.
So with all the Joker films, the Jared Leto one is in the DCEU with the current Batman, Justice League etc, while the Joaquin Phoenix film is in a different universe, and doesnt have to tie in to those films.