This distinction isn't nearly a bright line as you think it is, the US is essentially what happens when an EU-like structure has 200+ years to evolve. In the US, States have their own constitutions, legislatures, various legal systems, and almost complete autonomy with narrow exceptions. Around 30% of them operated autonomously as their own countries at various times. Americans have almost no contact with the Federal government, only State governments, and the nature of State governments, laws, and legal systems varies quite widely across the US because they evolved independently.

As a simple illustration, the US has no national ID because the individual States have sole authority for managing the identity of their citizens. Similarly, it is why all contracts denote the State whose legal system has jurisdiction over interpretation of the contract -- there isn't a meaningful Federal jurisdiction over such things. This is also why the US Federal government can't just "pass a law" to do things, what they can actually do is severely limited by what the States feel like allowing them to do. While the States don't flex their autonomy very often these days, almost all power in the US rests with the individual States.