Who would write the code for my hardware

I am beginning a project of having a accelerometer designed that will be built into a hammer ... I want the accelerator to collect data on the swings, acceleration , impact point, whether it is swung with the left hand or the right hand etc ... Here is my question after the engineer designs and it is prototyped and working who would write the code to get all this data ?  Does the builder write the code that gathers the information and a programmer write bridge code for the end device ? Or wold the builder only write the drivers and programmers take it from there ? How does this work ... can someone explain the how this process works and who would be responsible for what ?