Are Legendary and Mythical Pokemon the same type of Pokemon or not exactly?

Actually, they are not.
Legendaries are always one of a kind. On the other hand, mythical pokemons are rare like shaymine and darkrai. But they are not one of a kind.

I don't even know.

Mythical Pokemon are Legendary Pokemon except people in-game are unsure if they are real or not. Mythical Pokemon are also the ones you get in events.

No I don't think they are the same thing