All the recipes I've known for croutons required pre-made bread, fresh or old. Do restaurants always use pre-made bread as well? Is there ever a case for making croutons from scratch, and what would be a basic recipe?
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).
Most restaurants will use the bread that have not been served on the day's service as the source for their croutons.
I've seen restaurants at the end of the shift, slice left-over baguettes and put them in the oven for the next day service.
The bread can be home-made or bought from a bakery; either way it does not make a difference in the end product.