As I can find in Internet, there are several sites that a user can read about BSD systems. I found some good sources of information to me, while they can be to you too.
1. Man Pages: The Main and First document you need to read and no Internet is needed.
2. Duck it down: Use Duckduckgo or any other alternative to search about the topics you wish to grep.
3. Official Documentation: Not all documentations have search options but they are a good source of knowledge.
4. Forums, Blogs, Maillists, IRC
https://openbds.org/,
https://undeadly.org/,
https://www.h-i-r.net/,
https://xosc.org/,
https://www.openbsdhandbook.com/,
https://yetiops.net/,
https://bsdapps.org/,
https://maxiujun.com/,
https://unixdigest.com/,
https://www.freshports.org/,
https://forums.freebsd.org/,
https://securityf2.com/,
https://dbpedia.org/page/FreeBSD_jail,
https://blog.feld.me/,
https://www.freebsdhandbook.com/,
https://www.ixsystems.com/,
https://freebsdfoundation.org/,
https://srobb.net/,
https://journal.bsd.cafe/,
https://www.adyxax.org/,
https://etherealwake.com/,
https://openzfs.github.io/,
https://wiki.freebsd.org/
This are some of the links I found useful in my search for knowing more about FreeBSD and OpenBSD