There are a decent number of requirements placed on manufacturers, but the use of vehicles in the US are primarily regulated at the state level. Most states do not perform safety inspections vehicles, and even when they do, they are minimal in nature (e.g. do the lights work, are there holes in the exhaust, are the tires and brakes worn out?)

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.

The US has very lax automotive regulaions. The manufacturer has to affirm that the vehicle meets road worthiness standards (FMVSS) at sale and aftermarket parts aren't supposed to be sold or installed if they would take vehicles out of compliance. There are essentially no rules on driver assistance, so there's not much that would be prohibited.

> So in the US you can replace/add hardware to your car and install some open source software on it that takes over steering and braking, and this is legal?

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.