Honestly the thing that helped me most with that sort of stuff was just subbing to r/linux (now would be c/linux but this was like 4y ago now I guess…fucking hell time flies huh?) and reading up on stuff/asking questions for a few months before I switched. There’s also linux4noobs (both c/ and r/) for asking specific questions, it’s slower on lemmy but still can offer help and honestly I should probably make a post here about it again to drum up awarwness. Finally I’d like to add that many times communities related to specific distros will be more help than general communities, if you have a question about Fedora for example it can be helpful to ask in c/fedora over c/linux sometimes.
Now as to your questions here, just to kinda boil it down, the main difference between distros is the prepackaged stuff that comes with it, and the package manager. Package Managers are basically your “app store,” this is where you’ll get most of the stuff you need, for the rest, Flatpak is a package manager available for all linux distros, and some things will have .deb or .rpm files on their website. The ones you listed are indeed distros, Lubuntu is too however.
DEs are basically your UI. If you ever changed the launcher on an android phone back when that was a thing, it’s basically the same concept. Fedora and FedoraKDE are the same distro with much of the same stuff under the hood, but Fedora (Gnome) is more maclike and FedoraKDE is more windowslike in terms of user interface. For this reason Ubuntu (Gnome) and Fedora (Gnome) almost feel more similar than Fedora (Gnome) and FedoraKDE. You can install any DE on any distro for the most part, but in the beginning I recommend picking a distro by considering the DE first and the package manager second, and everything else (long time stable vs bleeding edge updates for instance) third. You can always switch later for free, the only investment is time and maybe an external hdd/ssd for backups.
Someone will 100% come correct me and argue this lol, but I’m just trying to kinda explain it in “doesn’t already know about linux” terms (so hopefully I effectively did that at least lol). If you have any more specific questions feel free to ask here or on linux4noobs, and of course you can always try these distros out for yourself before installing them! You can use a USB drive and a program like Balena Etcher, Fedora Media Writer (iirc available on windows), or Rufus to create a live boot disk, boot into it instead of your OS, and play around. There’s typically no persistent memory so everything you do will be reverted when you shut down (and all saved files will be lost), but it’s just for trying it out before you “buy” it so to speak (just don’t click install unless you want to install, of course). There are plenty of guides for that out there, it’s actually a step in the installation process for most distros.
Just to add for new users, find this info easily by typing “what to do after installing Fedora” into your search engine of choice, there’s many articles about it (and all of them have the same information for the most part).