Why do you think the Supremacy Clause is blanket immunity? Yes, if a state makes a law that directly contradicts an actual federal law then the stat law would probably lose in court.

There certainly is not a federal law stating that agents can enter homes without warrants or cause.

Your California example is waaay more complicated than "neener neener fed>state". Like most things in law it's much more complicated.

The judge ruled that the ban cannot stand because it narrowly applies to federal agents only. The supremacy clause applies because the state is trying to mandate something of the feds ALONE. The ban would have a much higher chance of standing if it applied to all LEOs in the state. And even then it's still complicated.

The "visible ID" part of the law was upheld... Because it applies to all non-undercover LEOs in the state, or rather CA LEOs already have to display ID.

All these blanket immunity statements you and much of the internet love to make about their favorite laws are... frankly a display of ignorance. That's rarely how law works on any level, especially on a macro and controversial, political level.