I'm intermediate Linux user and I've been interested recently in FreeBSD and OpenBSD operating systems. I want to learn advanced techniques of hardening Linux (both offensive and defensive security) and fundamentals of hardening FreeBSD and OpenBSD. I want to improve my Linux and BSD skills. I used many Linux distributions such as Ubuntu, Linux Mint, LMDE, Debian, Arch, and Gentoo. I've been renting cheap VPS for a few years to learn system administration.

I need any recommendations about books, online courses (I mean education platforms such as Coursera or Edx), professional YouTube channels about mastering Linux/BSD etc.

It's important for me to recommend in first the books about advanced Linux's hardening concepts (including both defensive and offensive security) and BSD (mainly FreeBSD and OpenBSD) hardening fundamentals because my grandfather is going to buy a book this month.

"Absolute OpenBSD V.2" by Michael Lucas is a great book. And "The Complete Privacy & Security Desk Reference: Volume I: Digital (Volume 1)" by Michael Bazzell and Justin Carroll is very good.

How about Hardened Linux from scratch?
http://www.linuxfromscratch.org/hlfs/