99% just monotype, but who knows if there are special mons a la gigantamax that do something different.Tera whatever is going to be bullshit. Is it confirmed that it only makes you mono type or is it still possible the leaks about triple types are true? In any case it boosts the chosen type AND gives you STAB right? So it's like a super Z move. I can't wait to use things like Psychic Volcarona, Iron Head Steel Dragonite, Aqua Tail Water Garchomp, Poison Jab Weavile, and so on. Counters are dead, long live max speed hyper offense!
To the best of our understanding the extra boost is only if the tera-type matches one of the mon's original types. Tera-Psychic volc would just get psychic stab, but tera-fire volc would get fire stab + extra boost, similarly for tera-bug volc.