Linux is the kernel. The kernel is something most users rarely interact with or understand. The kernel is basically interfacing with your hardware specifically and then creating an applications interface that all software can interact with.

So let’s say your computer has a small auxiliary board inside that your USB ports are connected with. Your mouse is plugged into that USB port. The auxiliary board has this random Infinion chip that creates the USB hub. The kernel’s job is to figure out how to use that Infinion chip and make a connection that is the same for all software to interface with. Your office suite or internet browser never needs to know how to interface with that infinion chip or any other specific hardware.

Windows has a micro kernel architecture. They publish a static spec for hardware manufacturers to write their own drivers for and the user must find and add them manually.

Linux is a monolithic kernel architecture. All kernel modules (drivers-ish) are included in the kernel itself and maintained by the community. The vast majority of hardware issues that happen in Linux are due to undocumented hardware; meaning there are no datasheets describing how the device works or how to program it. Undocumented hardware is due to seedy companies stealing IP and trying to hide it, and manipulating the market in an attempt to steal ownership from the end consumer while profiting from stagnation by selling old products while they lack engineering innovation and competitiveness in an open market. Soapbox over. The wonderful folks over at Debian are the ones that reverse engineer a lot of this stuff and make it work with Linux regardless of documentation.

Anyways, the Linux kernel is just part of the puzzle here. You can configure and compile your own custom kernel. Gentoo makes that quite easy to do for advanced users. Fedora has a nice guide I saw recently as well.

All CS students learn how operating systems work using Linux. There are lots of people who make their career in parts of Linux.

By itself Linux is basically just a terminal/command line. All the pretty graphics stuff requires other stuff like a DE.

The issue of initial scope complexity that you’re facing is really common. All of the distros have a purpose. They are not just branding or team sports. All of these distros are made by packagers that each have their own methodologies and preferences. Most of these differences can create compatibility issues, especially if you do not understand them. However, all of the packagers are building on top of a similar base of software.

When some one says you can just swap this or that outside of the packages configured by the distro maintainers, they are implying you have the same experience and understanding about the distro configuration and packages as the maintainer and a full understanding of a POSIX system, or they are just a fool, or happened to have success after following someone’s tutorial one time in a virtual machine. Few general users keep updating stuff like this over time. They just switch to a prepackaged distro that has the DE they want. The exception to this rule are savant types or people with no life or peripheral interests. Most of these people gravitate to Arch (and talk about it too if they are trolls), or use Gentoo where everything you do is configurable and made to compile yourself easily. The epic route is to do a Linux From Scratch build.

The best beginner’s route is to give up our ancient old mod a civic to pretend-street-race culture and just use the vanilla experience. Ubuntu is a lot less popular now. Fedora is the new Ubuntu, while Mint is the goto if you want a Debian derivative or to game. Fedora is pretty well dialed and handles secure boot well. SB is outside of the kernel, so is a thing that distro packagers either provide or don’t.

KDE is kinda like Windows. Mint has KDE and Fedora ships a KDE version too. I recommend just doing gnome, it seems a little funny at first, but it is well designed and intuitive. There are some headaches in the learning curve but it is not hard IMO.