How do Tera Types affect dual type Pokemon?

Question

I might be wrong in my understanding of Tera Types. Single type Pokemon become dual type (Flying Tera Pikachu for example)
But how do Tera Types work for Pokemon that are already dual type?
I'll use my Veluza for example. Its types are Water and Psychic with a Normal Tera Type. So when I Terastallize it, which of the original types would it drop?

Answer 1

Trainers who obtain a Tera Orb are able to use it to Terastallize their Pokémon, which makes a Tera Jewel appear above the Pokémon's head, causes the Pokémon's body to glisten like gems, and changes the Pokémon's type(s) to its Tera Type, removing all other types.

Terastal phenomenon (Bulbapedia)

Terastallizing doesn't work like Trick-or-Treat or Forest's Curse. Your Veluza would drop both its types and become a pure Normal type, and the same would be true for any Pokemon, regardless of how many types it had before Terastallizing.

However, you still have the STAB effects of your previous types.