It is part of their kernel. And there are a few different schedulers at play, you have the CPU one as well as the I/O one. Arch Linux has various different kernels (such as the zen one) that use different CPU schedulers (with lots of options in the AUR as well) and there are various settings you can tweak for different I/O ones.
I general you should go through the Improving performance wiki page on the details for these and even more performance tweaks you can do.
There is also pages for tuning things for better battery life for laptops as well that you may also find interesting.