The ending of Spider-Man: Across the Spider-Verse is a deliberate cliffhanger: Miles is stuck on Earth-42, Gwen is rallying allies, and the Spot is about to attack Earth-1610's New York.
What does Earth-42 tell us? Earth-42 shows Miles a world with no Spider-Man, because no one there was bitten by the spider that gave him his powers. That gives him a reason to reject Miguel's canon-event argument. Miguel's case is that canon events must happen, including the one Miles refuses to accept, the death of his father Jeff Morales. In my opinion, it is the cleverest piece of the film's argument, because seeing what a missing Spider-Man looks like lets Miles push back against Miguel instead of simply running from him.
How does the sequence run? The Spider-Society imprisons Miles, he escapes, and only then does he learn where he has landed. Gwen, meanwhile, goes back to Earth-1610 with allies to bring him home, so the closing stretch splits the two leads across worlds, a reversal of the way the film opens, when Gwen is the one cut off on Earth-65 and drawn back toward Miles.
How does it compare with the rest of the series? Spider-Man: Into the Spider-Verse is the film that introduced Miles and sits eleventh in story order, with this film twelfth. Spider-Man: No Way Home, released immediately before it, belongs to the MCU line, while both Spider-Verse films form their own separate universe in the series. In my opinion, that makes this the only entry in the series so far that stops mid-story by design, pointing straight at the next Spider-Verse film.
Why does the film end without resolving anything? The film ends unresolved because it is the middle part of a trilogy whose story concludes in Spider-Man: Beyond the Spider-Verse. The Spider-Society has locked Miles up and then lost him, Gwen returns to Earth-1610 to bring him home, and the Spot is set to attack. Perhaps audiences who want a complete story in one sitting will feel shortchanged, though that is a matter of taste.
For the full chronology around it, see the Spider-Man timeline.