The change from liquid to gaseous fuel is done with the flip of a switch. If it required major parts to be changed it would not be considered multi fuel. Some forklifts / industrial equipment are setup this way.

Multi fuel consists of leaving the standard gasoline carb in place then inserting a mixer or venturi at the carb air inlet. There are two solenoids used, one to turn off the gasoline and one to turn off the propane / nat gas.

When changing between propane and nat gas, the fuel mixture needs to be changed but this usually consists of turning a screw.

If an engine is setup to run only on gaseous fuel, a mixer or modified gasoline carb is used.

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As for priming the carb with gasoline, some regulators / vaporizers have a prime button or coil that is wired to the starter. Also, when the diaphragms harden with age they don't move so well causing starting issues. The device is a negative pressure regulator ( like a scuba regulator ) that requires suction to operate.