DC Extended Universe

One, several of the TV shows are in the same universe. While Supergirl is considered part of the "Arrowverse", it's an alternate reality that just crosses over with the other shows on occasion.

With DC's movies, the logic of having them take place in the same universe is that it's easier to sell a concept if there's a chance that an existing character like Superman, Batman, or Wonder Woman can show up in the middle of a lesser-known character's movie. It's what Marvel allegedly intended to do with the second Guardians of the Galaxy movie with the Hulk before that movie greatly outdid their expectations. (They threw Hulk into Thor: Ragnarok to give him something to do before Avengers: Infinity War instead.)

Apparently, there are plans in place for them to have low-budget, aueter-friendly movies as well that are completely self-contained, with the only known one being a Taxi Driver-inspired take on the Joker, set in the 1980s. But so far, everything indicates that aside from these occasional movies that pop up from time to time, everything else will be in one universe.

One other thing - Marvel has tried the "the movies and shows are in the same universe" angle. But with that comes a number of creative restrictions, such as the fact that the shows can't afford most of the A-List actors to play their characters on the show, or the general tendency for the movies to not really be all that bothered with acknowledging the fact that the events of the shows took place.

That's an easy explanation: those iterations of Superman and The Flash on their respective shows are not the same iterations as Superman and The Flash in the movies. That's the fun of the Multiverse - you can have different interpretations of the same character and not let that interfere with telling stories across other mediums. Think of it like how Tobey Maguire, Andrew Garfield, and Tom Holland play completely different versions of Spider-Man (even though those are Marvel characters).

Generally speaking, there's a sort of rule that DC seems to have about what characters they can and can't use on television to avoid confusing audiences. Deathstroke, for instance, was intended to be a part of Ben Affleck's version of The Batman, and as such, he was taken off of Arrow until recently, when Ben Affleck decided against directing the film and ceded creative control over to Matt Reeves (who is looking into different antagonists to use for his trilogy). And now, with an actual Deathstroke movie in development from director Gareth Evans, they're not going to use him again on Arrow for a while.

 

I think he could actually be a heck of a Darkseid if they took that route.

Anyway... Steven Spielberg doing a DC movie, even if it's not a superhero movie but a war movie with comic characters, is something that greatly excites me. As is the fact that WB is confident enough in James Wan that they're letting him produce a show in addition to directing Aquaman, which I've already heard a lot of good things about.