Hi,
Armbian does not provide the full source code by itself. It relies on external sources like mainline Linux or WiFi drivers.
When starting the compiling process these sources will be downloaded, for example from git.kernel.org or a mirror, patched and then compiled.
You can use the CREATE_PATCHES build option to automatically create patches from your changes to the source code which then you can apply on the next build run.
https://docs.armbian.com/Developer-Guide_Build-Options/
https://zuckerbude.org/armbian-using-create-patches/