Let's use Charizard as an example. It is 4x weak to Rock type attacks (2x Flying + 2x Fire). If Onyx uses Rock Slide on a Charizard, will the attack be 6x? (4x + 50% STAB)?
As you can see from the Damage Formula:
where Modifier is:
- STAB is the same-type attack bonus. This is equal to 1.5 if the attack is of the same type as the user, and 1 if otherwise.
- Type is the type effectiveness. This can be either 0, 0.25, 0.5, 1, 2, or 4 depending on the type of attack and the type of the defending Pokémon.
- Critical is 2 for a critical hit in Generations I-V, 1.5 for a critical hit in Generation VI, and 1 otherwise.
- other counts for things like held items, Abilities, field advantages, and whether the battle is a Double Battle or Triple Battle or not.
- random is a random number from 0.85 to 1.00.
You can see that Type effectiveness and STAB are both applied to the final damage.