The cross-over ended on Supergirl ("I hope this doesn't become an annual thing"
) and honestly this one was the best yet. SOOOOOO much fan service. I'm guessing Superman being gone on Argos with pregnant Lois could be the rumoured spinoff show (Tyler Hoechlin definitely deserves his own Superman show, one of the best casting ever and performance as well). And Lois wearing the same outfit as Margot Kidder on Superman - The Movie...
What was that hammer Lois had?
Crisis On Infinite Earth is a big cross-over I guess?
Crisis on Infinite Earths isn't just a big crossover. It's
THE big crossover. Really it was the first and (at the time) biggest publication-spanning multi-work epic tale that comics had seen. The in-universe reason was much the same as was set up in this show's crossover (the Monitor went looking for and found an assemblage of heroes from different universes to fight a giant reality-threatening evil), and the meta-reason for it was to allow DC to collapse its sprawling, confusing multiverse of different books and eras and realities into a much more compact package that wouldn't overwhelm readers quite so much. It turned out to cause as many problems as it fixed and its success made gigantic crossover stories into a cash cow to be milked mercilessly on an ongoing and too-frequent basis, but it was absolutely a milestone event in DC and comics as a whole
Crisis on Infinite Earths (Comic Book) - TV Tropes (some spoilers in the early description, more in the tropes list and other tabs)
Crisis on Infinite Earths - Wikipedia (even more spoilers here, right in the early couple of paragraphs even)
There's bound to be some changes not just because the shows can't possibly cover all the ground the comics did, but because I hardly think the tv version of this is going to end with a good chunk of the assorted series' rosters well and truly dead (Comic COIE slaughtered both C-list and A-list characters by the truckload. And many of them stuck for a long time. And several of them are characters important to several of the current CW series).
At the very least I expect that the crossover will be the impetus to finally realign the Supergirl show universe into the same reality as the rest of the CW series (except Black Lightning) and to give the assorted series writers free reign to tweak and alter the series to fix problems and disliked areas as they see fit with a nice and convenient "well the universes merged, so reality changed in all sorts of crazy ways" hand-wave.
I also don't expect we're getting a Superman show. I get the feeling that DC still wants to hold onto its primary heroes for a movie universe even if those versions of the characters are faltering and flagging to various degrees.