https://en.wikipedia.org/wiki/Vehicle_inspection_in_the_Unit...
Police can pull someone over for faulty equipment, but that would generally have to be an egregiously obvious defect.
From a federal level, the primary thing that is illegal to modify is emissions equipment (however, if a vehicle is registered in a state without inspections, even that is essentially unenforced)
In much of the US, you can legally drive just about anything with brakes, seatbelts, lights, wipers, turn signals.
US is a heaven for car enthusiasts, builders, fabricators, modders, etc
As far as I know, none of the other countries allow any kind, small or big, of modification to their cars. And, any changes like engine oil must be done by a pro. Too bad for them, they've killed a multi-billion-dollar industry themselves and thousands of jobs too.