Marvel's What If..? definitely stirred the ship and went on a different path as it has taken us to various alternate realities throughout the multiverse. Season 1 is ending on a high note with Episode 9 and it perfectly sums up the whole run of the canon show, so far, and it also promises more in the future that it would surely be a delight to all those following the series.
Warning! This portion may contain spoilers for Marvel's What If..?. Read at your own risk!
The Ending of Marvel's What If..? Season 1 Episode 9 Explained
Ultron with all the six Infinity stones really made a mess in the multiverse, sending his robot army throughout different universes, and Uatu is making his own way, his intervention, to counter the supervillain by choosing who would become part of the Guardians of the Multiverse.
Doctor Strange Supreme planned it out with The Watcher and they called on the following as the team to face Ultron: Captain Carter, Gamora who wields the two-sided sword of Thanos, Killmonger who is now the Black Panther, Star-Lord T'Challa, and Party Thor. Surprisingly, Natasha from the Age of Ultron universe was not called upon and chosen.
Still, the battle was taken to her universe and there, they faced Ultron. Natasha became part of the team and they finished Ultron off by putting the arrow where Zola is. However, just when they thought they already defeated him, Killmonger took the stones for himself wanting to have their own timelines back to normal.
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When Doctor Strange Supreme realized that their ultimate goal is just to remove the stones from Ultron so Doctor Strange just put Killmonger and Ultron who were in the middle of a joust between the stones and kept them frozen in time, like a bubble. He promised Uatu to watch over the frozen timeline and kept it in the pocket dimension where Strange is imprisoned in.
It became a pocket dimension within a pocket dimension. As long as Doctor Strange Supreme makes sure that neither Ultron nor Killmonger would reach the stones, so long as both of them are frozen in time, the multiverse remains at peace.
As for the others, they continue with the lives they left: Party Thor gets his date with Jane, Star-Lord T'Challa comes back to Earth and reunites with Peter Quill, Gamora is with her new best friend Stark in an amazing suit possibly for outer space, Captain Carter went back to when she was taken despite wanting to be in another time and she knows that is for the betterment of everyone.
As for Black Widow, though her universe was destroyed, it was considered that she wouldn't want to be back there being the only person alive there. So, Uatu had her placed in another, the one where her variant died because of Hank Pym, and he got reunited with Nick Fury on a SHIELD helicarrier while fighting with the army of Loki who wants to take over the Earth.
Marvel's What If..? Season 1 Finale is now streaming on Disney+.