5 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

How does this card’s <When Digivolving> effect work, exactly?
Your opponent chooses 1 of their Digimon. Next, delete all Digimon in play, from both you and your opponent, except for this Digimon and the Digimon your opponent chose. Then you gain 1 memory for every Digimon that was deleted by this effect.
Ruled Mar 28, 2024
Does this card’s <When Digivolving> effect delete Digimon in breeding areas, too?
No, Digimon in breeding areas cannot be affected by effects, so they aren't deleted.
Ruled Mar 28, 2024
For this card’s <When Digivolving> effect, can my opponent choose a Digimon in their breeding area?
No, Digimon in breeding areas cannot be affected by effects, so they can't be chosen. Additionally, note that this effect doesn't delete Digimon in breeding areas, regardless of whether they could be chosen or not.
Ruled Mar 28, 2024
How does this card’s <When Digivolving> effect work if my opponent has no Digimon in play?
All of your other Digimon besides this one are deleted, and you gain 1 memory for each Digimon deleted in this way.
Ruled Mar 28, 2024
When this card's <When Digivolving> effect deletes Digimon in play, can you and your opponent activate <Decoy>, <Armor Purge>, and similar effects to prevent their deletion?
Yes, you can.
Ruled Mar 28, 2024
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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.