Why is the supremacy clause considered to be the root of federalism?

You find a body in a kitchen with undigested eggs and bacon in its stomach. knowing the victim had a set routine they followed most mornings, what would you then check to most narrow in on the time of death? what time the victim’s alarm clock was set to that morning what time the victim goes to work that morning what time the victim usually ate breakfast what time the victim usually took a shower in the morning.