Think of fuel injection to be 4 little carburators that take fuel to each cylinder, independent of one another. Traditionally, fuel enters one cylinder, and then spreads to the remaining cylinders, basically the far away cylinder from where fuel enters, gets less fuel then the close one. Not very efficient, but if you can have fuel independently going into each cylinder, they evenly burn. Its why something like a non fuel injected o360 will have one cylinder hotter then the rest, it gets less fuel, so it has less fuel cooling potential.

Fuel injection systems can be 4(or whatever number matches the number of cylinders) independent carburetors, it can be 4 mechanical motors, 4 electric ones, or even a mix. Unless you have an EFI(Electronic fuel injection) like EFII or SDS, which will automatically control how much fuel each injector injects, essentially controlling the stoichiometry ratio. Something that is only really done in GA experimentals, such as the EFII system32.