Sleeping in a car isn't illegal necessarily, but there are increasing popup communities that settle in empty/low traffic lots and live out of their vehicles. Like most of America's problems, our politicans are sending police forces to "clean up" the effect, instead of trying to solve the cause.

Here's an article on Vehicle Residency https://www.thenation.com/article/society/homelessness-vehicle-residency-housing/

Sleeping in your car in public is not allowed in Germany either

In some places, parking lots are monitored by security and you'll be kicked out if you're sleeping in your car in the parking lot.

I see a lot of areas with "No Overnight Parking" signs or something similar, so they don't make sleeping in your car illegal technically, but you can't stay there over night.

Often times it's loitering charges, loitering being a fancy term for "being out in public whenever it displeases a person of authority". Sitting on a public bench, having a picnic, walking on a sidewalk, sleeping in your car, whatever, all of those can and will get you loitering charges depending on your exact location in the United States.

Then you have public intoxication charges which on paper are only supposed to apply if you're causing a public disturbance (despite disorderly conduct already being a charge for that, public intoxication just makes it more severe), but in reality it's mostly used to harass drunk people who couldn't get a ride home, or uber home, and decided not to drive while drunk. I wouldn't be surprised if you had a higher likelihood of getting arrested for public intoxication while drunk walking/public transporting home than of getting arrested for DUI while drunk driving home. But public intoxication and even DUI can also be used if you're sleeping off drunkenness in your car, while the car is turned off.

Less actually illegal and more that the lots are privately owned and the owning companies can have you removed from the lots of they don't like what you're doing.

In australia, it can be illegal too. Only 1 state has actually made it 100% illegal, that state being Queensland (which is a rather big state too, stupidly enough). Where I'm from (Victoria), it's not illegal at a state level, but some councils prohibit it in their local bylaws. In the rest of our states and territories the act of sleeping in your car isn't illegal, but some of the more affluent and snobby areas try to get around that by not offering anywhere to park overnight without permits or living in the area

It's illegal in the netherlands too