Embedded software source code is synthesized from a variety of sources, including:
* original code written fresh for the new system
* compiler or chip vendor library code or examples
* integrated development environment examples
* examples from Github, and SourceForge, or other on-line sites
* code from previous product generations, possibly written for another processor and compiler
All this code cannot just be tossed into a blender and mixed to a smooth consistency.
Somebody has to select existing source code, and make sure it is ok to use. Then he must modify, organize, and package it into objects compatible with the system hardware. Lastly, more software objects must be defined to tie it all together; and meet the system requirements.
This organizational effort is called embedded software architecture.