Nat gas has a lower energy content than propane but the concept of conversion is similar. Propane is held in a liquid form to save space. Liquid propane is run through a vaporizer / converter that is heated by engine coolant, the vapor is then run to a negative pressure regulator. ( Think scuba regulator where gas only flows when you suck on the mouth piece.)

Dual gasoline / propane isn't a big deal, the gasoline system is left untouched and venturi / mixer is installed between the air filter and throttle body. If you want gasoline, the fuel pump is turned on , if propane , a solenoid valve is opened.

CNG is Compressed Natural Gas. It is stored at high pressure but remains as a gas. ( you would have to use high pressure and lowered temperatures to have a phase change. ) The concept of engine conversion is the same. There are some conversion / factory systems that use solenoid type gasoline fuel injectors.

LNG is Liquefied natural Gas, this offers the dense energy storage of propane.