No dumb questions. There are lots of new terms to get used to. They are not the same.
Scratch is basically just a chicken snack. It is usually mostly corn and you just toss a handful or two on the ground so they can "scratch" around. It is not necessary but a lot of people like to feed it. It is NOT a chicken feed, just a snack.
Grit is little tiny rocks. Chickens to not have teeth. Instead the food travel into their crop and then through the gizzard. Chickens eat these little rocks which collect inside the gizzard and the strong muscles use the rocks like teeth to grind up food. They will not eat a lot of it and if they free range or have a large natural space they can find these rocks on their own. I keep it out because my hens spend most of their time in an enclosed run.
I don't know anything about ducks. But, for my pullets I don't start giving them scratch until they have moved outside.