-Sonic defeats Titan
-The End escapes the Titan (this isn't shown, only mentioned)
-Sonic and Sage go up into space, quick speech is given, press a few buttons
-The End is destroyed, Sage apparently sacrifices herself
-Ending cutscenes play.
If that sounds disjointed, it is. We get the ending, the ending song, a post-credits scene, another credits song, then a second post-credits scene. I'd read ahead of time that the ending was rushed, but that didn't compare to actually playing it. To give it something of a fair shake, I did watch the hard mode ending, which does involve something approaching a final fight, in that you actually control Sage and the Titan in a bullet hell segment that has the End actually have a presence, so to speak. I'm not replaying the game just to do this however, and while plenty of games have locked content behind certain conditions being met (hell, the series itself has done that), it does leave a particuarly bitter taste here, since the hard ending, while not perfect, does flow more naturally and give the game a better climax. Also helps that, without hyperbole, the End's monologue that plays through the fight may be one of the best end-villain monologues in a game, EVER (at least in terms of script, delivery's more up in the air). It also helps that the final shots of this fight, regardless of ending, are absolutely gorgeous, with Super Sonic appearing akin to a shooting star from the surface, and the meteor shower that results from the End's destruction. It's just that without the full fight, there's not enough buildup to this climax.
I'm going to throw in some random wider thoughts here before wrapping up, so on that note:
-I've commented before (I think) that a key theme, or at least motif, of Sonic Frontiers is death. That being said, I've come across the idea that I don't necessarily agree with, but want to give a fair shake. This ties in with the death theme, but there's the idea of The End representing death itself, that the Ancients were literally trying to outrun death by fleeing to Earth, but in the end, so can't outrun death forever, so they made their stand, were nearly wiped out, and all they could do was apparently delay death, not defeat it, by sealing the End in cyberspace. I'm sympathetic to that idea, but I think there's a hole in this theory, given that to best knowledge, Sonic and Sage successfully destroy the End, and even Sage apparently survives the ordeal, so apparently death CAN be defeated? To be clear, I'd still call death a clear theme/motif of the game as a whole, but I don't think the End can really be said to be an embodiment.
-Tangentally, there's the idea that the End represents order/entropy, hence why it sought out the Ancients' homeworld because of the Chaos Emeralds (chaos/life/disorder), and pursued them all the way to Earth. Maybe, but unless I missed something, while the game makes it clear where the Chaos Emeralds originated, it doesn't explain if they're naturally occurring, or if the Ancients created them (I'm leaning more on the former, since the Master Emerald is clearly a natural gem based on what we see in the story).