You have a fuel rail in an old mechinical system, after that they were controlled by a rotary injector pump, have one of those on my case tractor. electronic enjectors have oil pressure to fire them around 3500+ psi, the same amount of pressure on each injector, the fuel is run into the injector by the fuel pump, the electric switch on the top of the injector controls firing and length of firing. Have had my powerstroke apart several times. And the system is the same on all the new big trucks.
Modern diesel engines are mostly all going to high pressure common rail systems. Injection pressures are in the range of 14,000 to almost 30,000 psi.
Anyway, in a dual fuel application, you basically cut the diesel quantity back to about 10% and add the remaining 90% of the desired fuel with gas. The diesel is required to control the ignition timing. It works very well.