Every car is different and may require specific air pressure for your tires.
This can be found on the frame of your driver’s door. This plaque will also tell you the recommended tires you should keep on your car per the manufacturer.
This usually applies to your everyday driver that probably doesn’t have custom tires or rims that would null the recommended pressure for the car.
Edit: WHY you should know! (I forgot to add thank u @_MoveSwiftly!) So keeping your tires inflated is important because a) it keeps you safe. Properly inflated tires will have better stopping power B) wear and tear on a tire is lessened and wear and tear on other aspects of your vehicle that require good balance like shocks and brakes. c) in most cases having your tires inflated to the proper psi/bar will help you hit the MPGs, horsepower and acceleration advertised for your car.
Depends on the car and spare. On my car the spare is a much more thin tire compared to my regular ones. It’s meant to act as a short-term fix to get a proper tire back on. This type of spare isn’t meant to be used long-term.
Since it’s a different form than a typical tire, many spares (mine included) are almost double the PSI (~60) than my typical tire.
I’d say just make sure you check on the tire itself! It’s definitely likely it’s different from the main tires for many makes and models.