A model attacks with all weapons it's equipped with.
This doesn't mean that it attacks with all weapons listed in its profile, though, because you're bound by the restrictions listed on the warscroll.
Example Bloodreaver:
Some units of Bloodreavers fight with Reaver
Blades, while others prefer to use Meatripper Axes.
So the whole unit has to choose with what weapon it's equipped with and can attack only with the equipped weapons.
Example Orruk Megaboss:
They enter battle armed with a huge Boss
Choppa in one hand, while their other
hand is sheathed in a Rip-tooth Fist made
of heavy iron.
Yep, he has both weapons and can attack with both.
Edit: mounts like the Dracoth are always in addition to any other weapons and the weapon profiles list the mount's name before the attack name, e.g.
Dracoth’s Claws and Fangs