Heh, he started the GNU project, which was aiming to provide an open source, UNIX-like system. For this, he created one of the first open source licenses, the GNU Public License (GPL), which has a controversial, viral clause that guarantees users access to the source code of any GPL-licensed software they use.
Anyway, he started by implementing the userspace tools commonly found on UNIX systems, but GNU's kernel never matured (HURD exists, but mainly as the butt of many jokes). Then Linus took the GNU tools and ported them to his kernel, hence "GNU/Linux" = GNU tools on the Linux kernel. You can use the kernel without the GNU tools, such as Android does.
Linus also used Stallman's license for his kernel. Any device drivers that are included in the kernel source tree must have this license. This is a high bar that some, such as Nvidia, have not been willing or able to clear. Even after the doc dumps, Nvidia continues to have their proprietary "out-of-tree" drivers and provides no official support for the open source in-tree Nouveau driver.