Necrozma can be found via Dynamax Adventures in the Max Lair in Sword and Shield.
Necrozma can be found in both versions, but only appears after the They Came From The Ultra Beyond! adventure is started.
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The player then finds an adventure description about Ultra Wormholes that Peony doesn't even recall writing. Showing it to him will make finding and catching Necrozma at the Max Lair the new adventure, calling it They Came From The Ultra Beyond! (Japanese: 伝説！空を覆うウルトラな穴伝説！！ A Legend! Ultra Legendary Cave That Covers the Sky!!).
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Do note that to get the They Came From The Ultra Beyond! adventure, you have to complete the previous adventures (catching the fused Calyrex, legendary Titans (excluding Regigigas), and the Galarian Birds). Additionally, the Max Lair is in the Crown Tundra, which is paid DLC.
Hope I helped.
Edit: I forgot to include Ultra Sun and Ultra Moon, but that info is found on the site.
