What do dentist use to hold open your mouth?

Depends on the severity of the procedure. If it's something simple then often they don't use anything and rely on the people keeping their mouth open on their own. When more complicated procedures are happening, then they use a Cheek Retractor, which is a fairly simple device that holds the mouth open and enables them to work even if you're sleeping due to anesthesia.

They use a cheek retractor, my aunt is a dentist so i asked her