Croutons from scratch?
I can't speak for all restaurants, but those where I've been served obviously home made croutons also served similar bread with other dishes. Bread is cheap and readily available. Why would a restaurant bother doing anything more difficult? If you can get day old bread from a bakery it's even better, because that's really cheap, and works at least as well as fresh

As for making them from scratch, I would indeed start by making a loaf of proper - yeast-based - bread (I like soda bread but the texture doesn't work here and it would fall to bits).