Open Source and Free Software

These variants include OpenBSD, FreeBSD and Linux to name just three popular ones.
The applications, including command shells are very similar, sometimes identical across versions.

The kernels are different. You would see this in the management of them (startup scripts, package management and system administration)

Linux does itself have two main branches: Debian variants (notably Debian itself and Ubuntu) and RedHat variants.

To your point, these are mostly user-installed on intel (PC) hardware, though there are some laptops and desktops that come with Linux installed (System76 and Dell come to mind)

I recall that any BSD operating system is user-installed. So, they are not for beginners.

Historical note: BSD refers to Berkeley Software Distribution, which took its inspiration from AT&T's Unix. It was close enough that AT&T fought it lin court long enough that Linus Torvalds got tiired of waiting and wrote his own Unix version (Linux) from scratch. BSD eventually got out of that quagmire.

OpenBSD is a version of BSD that's audited line by line before a version is released, with total emphasis on security. Out of the gate (until people change anything) it's the most secure operating system commonly available.

If there are community college courses on "Unix", they are likely to be using Linux.
But you can look at open courseware (I am thinking M.I.T. Open Courseware) and see if something gets your interest.

If the RaspberryPi computer interests you, it mainly runs Raspbian, a version of Debian Linux.

Happy Computing!