Um… I will assist???
The most simplest answer I can give is that they are separate universes and worlds.
Phoenix’s Joker is it’s own thing. It has no correlation to the DCEU or the Arrowverse, if you were wondering about that.
As for the Arrowverse, during the Crisis Crossover back in 2019 Kevin Smith hosted some sort of aftershow thing where guests would come on and do interviews and such. Some were writers and producers from the shows, some were actors like Kevin Conroy, etc. Anyway, on the show, there was somebody who made up a chart of the multiverse and which shows were set on those worlds, hence why I know that Titans and Doom Patrol are set on 2 different earths. Titans is set on Earth 9, and while there is a version of the Doom Patrol on Earth 9, the actual show Doom Patrol takes place on either earth 11 or 19? (I can’t remember exactly, okay?). This might even be the reason why we don’t see Jane on the Titans and DP crossover episode, and why the DP show don’t acknowledge Gar being on the team, its because the earths have a different timeline and events. The Batman 66 show is set on Earth 66, the Tim Burtonverse is set on Earth 89… and that’s all I can remember if I’m honest.
I hope this helps, and I’m sorry if I made this confusing, but this is the most simplest way I can give an answer and sort it out for myself. Some of the Arrowverse shows are not set on the same earth that Flash or Arrow take place (I think even Stargirl is set on a different earth but I could be wrong, and don’t ask me which one because I don’t know). The DCEU is it’s own timeline, and I think spin-off movies (such as BoP and Suicide Squad) technically are apart of the DCEU, but the filmmakers are specifically making them to where you don’t have to necessarily watch the first movie (especially if it’s bad). To those that hate it when these characters are different from their original source material, all I can say is that the multiverse and different timeliness are the in-universe explanation for that. As for what goes on in the moviemaking reasons, we will never know.
I hope this helps? At least a little bit?