Sweet and sour sauce is a thickened version of a Chinese marinade for fatty pork. Western recipes come from Canton via Hong Kong, and usually have foreign ingredients.
Combine a cup of apple cider vinegar, a half cup low-sodium soy sauce, a tablespoon of sesame oil, a half cup ketchup, and three-quarters cup of white sugar in a saucepan. Mix well, add a half cup pineapple or orange juice, a slice of fresh ginger, and bring to a simmer. To a quarter cup of cornstarch, stir in a teaspoon of red pepper flakes, and enough water to make a thin paste. Dribble into hot mixture; when it begins to thicken, set aside to cool. Remove ginger before serving. Or not.
