There are national emissions standards for manufacturers, and some states have additional, more restrictive measures. California basically sets the national standard by being more restrictive. If a manufacturer meets California's requirements they will be able to sell everywhere else in the US as well (this is a broad generalization with caveats but this is a Lemmy post not a thesis)

For vehicle owners vehicle registration requirements vary greatly from state to state. Some states include annual emissions checks with random testing as well. Failing these tests means your vehicle cannot be legally driven on the road. Other states only inspect when the vehicle is initially registered, and only check for the minimum items, such as working lights and a clean title (vehicle titles can be declared "salvage" if they are involved in an accident, and the vehicle goes through a more rigorous inspection to have the title cleared).

The US has been in regulatory capture for… probably longer than I've been alive. States like California are doing the heavy lifting when it comes to regulation. The national government is so beholden to corporate interests that the only reason there are still national regulatory agencies with enforcement authority at all is because the national government is too dysfunctional to change anything. This is life in a burgeoning oligarchy.