OpenBSD is known to provide correct and extensive documentation, compared to e. g. GNU/Linux. Is there a technical overview, for a developer, who would like to start contributing code to OpenBSD? I usually mix with low-level aspects of OSes near HW. There is a Kernel Newbie program or a few books like Linux Device Drivers. Is there something similar available concerning OpenBSD?
Unfortunately, as far as I know, there isn't similar books for OpenBSD. Mostly this information is shared in OpenBSD's handbooks, man pages and mailing lists. Definitely recommend reading into OpenBSD's man pages since they can offer great way to dive into for example system (man 2) and kernel calls (man 9).
Thankfully Stefan Sperling gave a great introductory speech couple of years ago on how to get started with OpenBSD kernel development, see slides at https://www.openbsd.org/papers/eurobsdcon2017-device-drivers.pdf and video itself at https://www.youtube.com/watch?v=W5qhWw07qpU.
Hopefully with these you can get started!