For Kodi and most of your other multimedia needs, you'll want to visit rpmfusion.org
and add their -free and -non-free repositories.
I'm no expert on the packaging systems, but the gist is along these lines: Software (aka Gnome Software) is the default GUI tool for managing software. dnf is Fedora's default package management tool, replacing the older yum. Both dnf and yum are analogous to apt on debian systems, while "Yum Extender" looks like would be analogous to Synaptic on a debian or ubuntu system. I've never used them, but the "plain version" would appear to be a gui frontend for the older yum, while Yum Extender-DNF is a frontend for the new dnf.