Bingo. Each state is like a country of its own.
Close, but not quite. It could have been that way if not for the civil war.
Each state is under obligation to follow Federal rules (no “interpretation”), federal currency, etc.
The USA should be looser in many respects - if New Hampshire wants to have abortions and Texas doesn’t, there shouldn’t be a federal common law forcing either state to do what its people don’t want to do (btw I’m pro-life)