This Theme is played, when you enter the Terarium the first Time and a Cutscene plays.
From Pokémon Scarlet/Violet The Hidden Treasures of Area Zero Part 2: The Indigo Disk

"Recorded InGame with a Portable Audio Device in HQ connected to my Switch."
PS read the Channel Description... You get it what I meant if you know these other two special YouTubers LOL

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