The early electronic FI systems on cars, circa 1980s, were pretty simple. They had a lookup table indexed by throttle position, air temp, RPM etc. The value in the look up table was a time interval to hold the injectors open for the correct amount of fuel.
There was also an injector open time based on RPM. This was a manifold injection system where all injectors opened at the same time.
The entire electronic FI system was powered by a 40 pin printed circuit board, without any computers. It was real easy to troubleshoot with a multimeter.
Almost as simple as a carb, but way more fuel efficient.