In Florida, it varies by department. Some automatically impound, but in some places an officer may tell the arrestee “hey, you’ve been cool with me. Is there somebody who you can call who can come get the car?” If they have somebody who can arrive within about 20 minutes, the backup officer will wait with the car. Alternatively, it’s towed.
(If there’s a passenger, they are permitted to drive, but they usually have to blow into a portable breath test, and the passenger is almost never sober if the driver has been drinking. So they have to get an Uber or have somebody come get them).
Now, whether a friend comes to get the car, or it’s impounded, is important. If the cops impound the car (even if that means that a tow truck takes it away), the cops are permitted to conduct what is called an inventory search. The purpose is not to look for evidence of crime, but to document personal possessions (you don’t want somebody complaining that their Rolex watch that was in the glove compartment went missing). As such (and unlike a criminal investigation that results in a search of the car), there does not need to be any reasonable belief of evidence of crime for the cop to do it. The justification is simply that the car is being impounded.
Of course, if in the course of inventorying any personal possessions in the car, the cops find evidence of crime, they are entitled to use it.
So hopefully your buddy answers the phone when you get pulled over and arrested at 2 am.
Going back to this federal weed law thing…
In Florida, an officer might be theoretically permitted to conduct an arrest on your citizen arrest theory - an officer outside his or her territorial jurisdiction is indeed entitled to conduct an arrest under this doctrine, so I can imagine applying it to the “federal jurisdiction”.
BUT…an officer can only make the arrest, since that would be her right as a citizen. The officer cannot conduct an investigation, as that would be acting under color of law, something a citizen cannot do.
So, it would not be a means to enter into a criminal investigation. Basically, as already described, the officer would have to make an arrest, then locate some federal agent to complete the detention - you can’t throw them in the local jail if they didn’t commit a state crime.