How does Terastallizing interact with type changes?

Question

If a Pokemon changes its type before Terastallizing, does it get STAB for its original types or the types it had immediately before Terastallizing? Are Terastal Pokemon immune to type changes? If no, what types get STAB after a Terestal Pokemon changes its type? Please do not question my curiosity.

Comments

By changing it's type, do you mean by something like Protean or Libero?

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yes (filler)

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What about moves like Soak?

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also yes

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Hiding my answer for now, but I'm probably going to revisit this question later when I have the chance.