The 10th Amendment explicitly says that any powers not given to the Federal Government by the Constitution are the right of the state or the individual. Thus, the Federal Government can only act in a way that is justified in the Constitution. States do retain a number of rights that the Government cannot legislate on and this leads to various states with various laws. The Federal Government can build roads and canals if such a project has a "interstate" function. With the Interstate system, typically the states maintain the Interstates in their jurisdiction and bill the Federal Government for the maintenance (this is how we have a national drinking age at 21. The feds refused to pay any state for the interstate if they didn't comply with 21 as the drinking age. And drinking ages in the U.S. are state laws... but they are universally 21 because any lower will not get those wonderful federal dollars.).
In addition, states can enter into compacts which are agreements between two or more states on matters of law that the Federal Government cannot enforce (the most famous being that a drivers license issued by any state is valid as another state's license. If Maryland issued the license, the Virginia police officer pulling you over will count it as if it was a valid Virginia license. This doesn't excuse you from breaking the radar detection law. You have to follow the law of the road in the state you are in. All 50 states have signed this compact).
In the beginning, each state was effectively a nation unto itself (Texas was a real country before it's annexation). As part of joining the United States, this country agrees to cede certain powers (such as the ability to make war, conduct foreign negotiations, hold treaties, print currency and regulate interstate trade, ect.) to the Federal Government, which decides the policy in these matters for all states.
Implied Consent is a legal principle stating that a first responded must ask for consent to attend to a victim EXCEPT in cases where the victim is unconscious, at which point the consent to perform life saving operations within the first responder's scope of knowledge is implied.