The vast majority of Samsung's phones come with 128GB of storage on the base model, including many entry-level Galaxy A devices. Flagship phones, like the S21, can be equipped with up to 512GB of internal storage.
While in the past, Samsung users have had the luxury of seamlessly expanding their storage through a MicroSD card slot, that's not the case anymore. You won't find a slot to boost your storage on any of the three Galaxy S21 models, and it's unlikely to make a comeback on newer flagship models as they're released. Those investing in one of Samsung's A-series phones are in luck as those devices seem to be keeping the slot (for now).