They can both refer to the same thing but are used slightly differently. If you’re having a meal that has an entre, main and then dessert then you’d refer to each of these as a course in the meal. Dish is more used for when you have lots of plates with different food on the table at the same time, each one is a dish. It’s also used for grouping similar types of food together on a menu for example, Pasta Dishes and Fish Dishes

Soup salad is familiar

Both are OK. Which one you choose may depend on context. “Course” is a little fancier than “dish,” usually. For example, I would probably use “dish” if talking about an informal potluck get-together: “John brought a salad dish for the potluck dinner,” but might use “course” if talking about a dinner at a fancy restaurant. But I think the two words are usually interchangeable.

Salad is a dish consisting of mainly vegetables.