It will spit out a rootfs, or even a block image for a disk, which might make it _look_ like a single application. You will probably update your system by flash

It will spit out a rootfs, or even a block image for a disk, which might make it _look_ like a single application. You will probably update your system by flashing that entire rootfs/image to your target device as if it was one application. However, it is still Linux under the hood, so there is a kernel + whatever other applications you want running on there, all as separate processes. You may also be thinking of busybox, which bakes an implementation of most of gnu coreutils into one file to save space. Many symlinks are often created to that sesame file, and when invoked the process can check what name it was invoked with to determine its behavior.