From what I know of, Galaxy A model with an s is a slightly upgraded version of those model without an s. For example, A50 and A50s. A50s has an improved camera.
When a product launched is between 2 products, the S is used to differentiate it. The Galaxy A21S gets the S moniker because it's slightly better than A21 but lower than A31, and there are no numbers left, so S is used
