Every modern operating system has some sort of startup routine. Even DOS had the old AUTOEXEC.BAT and CONFIG.SYS. The startup system defines what is launched on system startup, under what conditions, and in what order. DOS did things the simple way, having a file that was read on every boot that would run commands in whatever order they’re placed in the file. It is indeed simple, but like much of DOS, not nearly powerful or flexible enough to cut it these days.