The "right" way to do it is to use a machine which pulls out the (usually) R 134 and creates a vacuum within the system. Then if there are no leaks, a full fill of refrigerant is added by the weight listed for a particular car while the before and after the compressor pressures are observed on a gauge set. Often the receiver dryer is changed as well, especially if the system has been "open" for a long time, that is, due to a major leak which left zero refrigerant pressure and allowed outside air (with humidity) in.
Car systems must have flex hoses and use threaded fittings, the flex hoses are often lined with teflon to make it less likely for R 134 to migrate through. (There was less of a problem with R 12.) Auto compressors have a weak spot at the seal where the drive shaft enters. When it wears a very slow leak develops there. Appliance systems have an electric motor driven system which is entirely in a can INSIDE the pressurized loop. If the compressor leaks a bit it's just into the system, not out into the atmosphere. (Motor failures, however, can make a mess of the entire system.
As another post pointed out, appliance lines are metal and soldered.
If your system is just not cooling because of a small leak causing refrigerant level to be low (but not low enough to trigger the low refrigerant shutoff) you may notice uneven cooling with cold air from some vents and warmer air from others. That's when adding a small amount yourself can bring it back to full operation. I've seen as little as 6 oz from a 12 oz can do the trick. Follow the instructions (engine running, compressor on) and when you get cold air evenly from all the vents, stop adding. You can't fix a system that isn't cooling at all this way, it needs the attention of a shop with the right equipment. A full refill requires a specific amount of oil of the right type to be added along with the refrigerant. Leave that to the pros.