The first file loaded is?

[Decker, Ross]

I'm just starting my adventure into BSD along with C++.  I know that
this OS gives you all the source files that are used in the OS. What I'm
asking, is what is the name of the first file that is loaded and where
is the source file for it? I want to start with the very first line of
code and see if I can step my way through the entire boot process. There
are many things that I see during boot up that I have no idea what they
are (SMAP Type....) that aren't that Google Friendly.

So if anyone knows the names of the source files and their locations for
the boot sequence could you please post them..... SUPER thank you's!