The third step performed by the software is to start any executables that you want to be running. The OS does this by reading and processing information stored in the startup script — a sequence of commands stored within the image. The format of the startup script, as well as the buildfile that it's part of, is documented in detail in a variety of places in this guide:
Basically, the OS processes the startup script file, which looks like a shell script. In the startup script file, you'd specify which executables should be started up (and their order), the command-line options that they should run with, and so on.