Living out of your car seems like that perfect fairytale
You wake up in your backseat sleeping bag and you take on the day behind the wheel, but doing this may be a little more complicated than it's made out to be.
If you drive a pickup truck, this is helpful off the bat. If you have a cab roof to go over the bed, you can sleep in there while well-protected from the elements. In SUVs and vans, as long as you have the back seats down you can sleep the same way. If you drive a sedan, compact, or subcompact car, you may run into issues pertaining to space, but may be manageable depending on your situation. Using blankets, pillows, sleeping bags, maybe even a mattress pad, etc. you can sleep comfortably in your car. This, sadly, is where the simplicity ends.
Where will you shower?
If you can find a local rec center or recreational swimming pool, there are showers there; but when traveling those can sometimes be few and far between. You can find showers at campgrounds sometimes, but you also run into the same problem there. Restrooms for purely using the restroom can be found in any restaurant, and when in nature you can/may have to improvise. When it comes to eating, yes, fast food is an option. If you enjoy making your food, you'll need a portable grill and the supplies to get it going and/or maybe a microwave that you can plug into an outlet in your car.
Pertaining to where you can park, Road Trip America suggests anything from Walmart or casino parking lots to city street parking
Anywhere where parking for extended periods of time is legal, you can park there. But if you have a job, keep in mind that you should probably be close to your place of work. If you need to freshen up before heading there, then it's a good idea to have alarms early enough so that you can drive to the places with showers and such so you can freshen up and make it to work on time. If you work from home (aka online), this may present some complications as well. You'll need outlets for an internet router and to charge your devices. Many cars have USB and USB-C ports, and some even have normal household electrical outlets; but the problem is that to use these effectively, your car needs to be on. When working for multiple hours on end, this can use up a lot of gas which will be costly.
Mail for bills and such could be complicated, too
You won't have a concrete address for bills, mail, and packages to be sent to. You can get a PO box that will solve some of the issues, but some delivery companies won't deliver to these.
This isn't all-encompassing- there may be more things that you need depending on your situation and what region of the world you hail from, but these are a few things you need to consider when you think of living out of your car.