The culture is the sum of the Backgrounds within it.
Considering outside of specific cases, which I made allowances for, every culture (lower case C) can have the same Backgrounds (capital B game object) then that would mean potential each culture (lower) is the same, as they are a sum of Backgrounds (capital).
Obviously, that is nonsense.
The correct conclusion, is that culture in 5e, is a lore/fluff consideration, largely divorced from any mechanical weight, even less important than Alignment (capital A) to the humour of all.
A Background (capital) is a game object. There is no Culture or Cultural Feat game object(s) and as such they are meaningless terms within the context of the discussion. They are not real, within 5e.
Species - Your biological, innate attributes. Such as Breathing Water, running Faster, or seeing better in the dark.
Class - Your 'profession' as an adventurer. Fighter, Rogue, Wizard, Cleric.
Background - An ASPECT of your characters background (lower case b). No mechanical ties to culture (lower), and no lore ties, unless specifically called out in the fluff.
Feats - Localized mechanical objects that grant a subset of rules/actions.
Those are your 4 options.
The astute will notice that Culture, does not exist within the context of 5e, as it is not a Game Object, and as such cannot contain other Game Objects.
Character, is an object, mechanically defined by Species (formerly Race), Class, Background, and Feats.
That is the totality of what defines Character.