First, no, I don't think that Linus Torvalds himself does still write much code that goes into the kernel. But what he did some years ago was writing a source control management tool as he wasn't quite happy about the existing SCM Tools at that time. If you want to hear his own words, you can for example watch one of his great talks: https://www.youtube.com/watch?v=4XpnKHJAok8
So basically it is his invention of git, that he mainly did for exactly that purpose, to manage the release cycle of the linux kernel.
And that's what actually made the release frequency of stable kernels that fast, we all experience since major version 3.
When you are good at git you can address many objectives, known today as continuous integration and continuous delivery to get small (one-person) up to huge (several thousand) distributed development groups together, to continuously deliver working software.
But of course, Linus Torvalds does none of this on his own. He is just the last man who signs a mainline kernel release on http://kernel.org.
See the git's user manual delivered with git, or here, for an example of how to work together in huge software projects.