Linux was created as an enhancement of Minix, a pedagogical tool created by Andrew Tannenbaum in his Operating System classes, as a scaled down “toy” UNIX-like OS that students could do useful things with in one semester. One of his students (Linux Torvalds), completely missing the point, filled in the missing pieces and created the Linux kernel. It became the center of the GNU project and was protected by the GNU Public License, which prevents its free and open use in commercial products. If you do use GPL code, it can “infect” your code and make it open source too, if you aren’t scrupulous about all of the details (and the GPL license is a moving target). There are even “GPL Police” that sue commercial companies that violate and aspect of this license.
Linux got IPv6 support a bit after FreeBSD, via the USAGI project of the same WIDE project. There was a native IPv6 stack for Linux at one point, but I believe most Linux distros are using the USAGI network stack today.