I see this. Where the source of libc6-dev-i386 is from?
$ apt-file search /usr/include/sys/socket.h
libc6-dev-i386: /usr/include/sys/socket.h
https://packages.debian.org/search?keywords=libc6
https://www.gnu.org/software/libc/sources.html
Is it from git://sourceware.org/git/glibc.git?
It tells you on the page for the package on the right hand side. It gives the origin as the GNU.org, with links to the files needed to build it linked there as well. The .orig.tar.xz is the original source package from the GNU the other two the Debian files used to build the packages with the changes made. This is the same for all packages, lookup their pages and you get the same information.
https://packages.debian.org/sid/libc6