If I have weapons on a bandoleer like darts or daggers, do I use the "retrieve an item" action or the "draw a weapon" action to use them?
This leather belt is worn over one shoulder and runs diagonally across the chest and back. It has small loops or pouches for holding eight objects the size of a flask or small dagger. You can use the "retrieve a stored item" action to take an item from a bandolier. You can wear up to two bandoliers at the same time (any more than this and they get in each other's way and restrict your movement).
RAW it's Retrieve a Stored Item.
That being said, your DM may well rule it "draw a weapon" because the dagger's Weapon quality should probably override its categorization as an Item for how it's drawn.