Device drivers are in the kernel.
Linus wrote the original Linux kernel by himself, and remains very much involved (last I heard about it anyway) but now there are thousands of people developing for the Linux kernel. These folks include employees of companies like Intel who write drivers for their devices for the Linux kernel (and get paid to do it).
Note that one driver can work for many different devices. For example, i915 is the kernel's Intel CPU driver. There doesn't need to be a driver for every single different Intel CPU.
Loosely described, Linux kernel development is a huge, widely distributed project overseen by Linus and other core developers, with people contributing as their full time job, part of their work, as a hobby or project, all communicating through mailing lists. Anyone can write a patch for the kernel, patch the kernel, and use and distribute their patched kernel. But if your patch is so great it really ought to be in the kernel by default, with some feedback and help from others, it might get merged into a future kernel release. This open and diffuse model of development, with many minds and perspectives on the job, accounts somewhat for how the Linux kernel manages to support such a wide range of hardware. Here's the documentation for kernel development.
As chilli555 pointed out you can download the kernel source code and find device drivers. To do this, enable source repositories, create a directory in your home and enter it, and from there run (
sudo is not needed):
apt source linux