The concept of federal preemption can be tricky, so I’m going to clear it up real quick.
Federal preemption is outlined in the Supremacy Clause of the US Constitution. That clause says that laws made by the United States “in pursuance” of the Constitution are the supreme law of the land which means that acts not in “pursuance” of the Constitution are not laws at all. So, when a state law that is considered “unconsitutional” conflicts with a federal law, the federal law trumps the state law.
A good example of federal preemption has to do with the National Labor Relations Act. The NLRA is a foundational statute of US labor law which guarantees basic rights of private sector employees to organize into trade unions, engage in collective bargaining for better terms and conditions at work, and take collective action including strike if necessary–basically, it allows you and your coworkers to band together and raise hell when you feel like you are getting shafted at work. You can fight…for your right…to party–or whatever. If a state were to attempt to write a law that limited or restricted your ability to unionize or strike, that law could be contested and overturned by the Supreme Court.
Generally, federal employment laws constitute a minimum level of employment regulation, while state laws are more strict. For example, the federal minimum wage is $7.25 an hour, but you live in Connecticut where the minimum wage is $8.25. In this case, your state minimum wage prevails. Federal labor laws dictate that you are to make at least the state or minimum wage (whatever is higher)…so you get $8.25 an hour and a nice house made of cardboard boxes to live in under a bridge because that’s all you’ll be able to afford in the overpriced shithole that is Connecticut. (I’ve been there. No offense, but you couldn’t pay me to live in that crapbucket state.)
In the absence of a state law, you refer to federal laws. So, if your state doesn’t have a law that prohibits or allows your salon owner to force you to work 55+ hours a week without overtime pay, you would then check the federal labor laws.