well gnome software and epiphany app stores just work.
click, install, done.
they provide an option to pick the source to install from (package/flatpak/snap), but they both automatically pick the best one for you.
Debian/Ubuntu almost never break on updates (unless you mess with the PPAs too much), but at a significant cost: some packages and software (especially desktop environments and system packages) being 1-2 years out of date.
you mentioned that file previews are broken for you, thy should just work, unless some component it terribly broken or missing…
also about the last part, package name usually matchess the name of the command, so for example if an online guide tells you to use the ffmpeg command and it’s not found on your system, usually that means that you have to install a package called ffmpeg.
some package managers and command line shells provide more helpful error messages, like: command X was not found, but here are some packages that provide this command, do you want to install one of them?
by the way, you mentioned that you tried using Fedora. common source of frustration is beginners trying to use apt on a system that doesn’t support or use it (apt is only used in Debian, Ubuntu, and their derivatives). Fedora uses dnf instead.
…but, as a beginner, you shouldn’t even worry about this, as most distros provide easy-to-use, graphical app store applications that can automagically install apps (from your package manager, Flatpak, Snap, etc, picking the source automatically if it’s unavailable in one of them) with a single click.
you don’t have to use all of the app containers things, pacman, apt or whatever your distro uses is often enough.
if you don’t have previews at all, your system is completely broken and fucked up if you get a command not found, well you just need install the missing tool…