There’s some confusion here - with Windows, an installation can leave instructions to reinstall itself, registry entries (surely Registry is the most loved aspect of Windows) and make things very tightly integrated. Certainly with XP and Vista I had nightmares about this.
Software (like Mozilla) will deposit ‘user’ files and folders in your /home. They aren’t ‘installed’ yet they will be used if you reinstall later. To detect errors, sometimes renaming .config folders is enough (.mozillaBAK for example).
To get more help with using pacman, go to your terminal and type pacman -h
it gives you clues…
Try pacman -h -R and see further down the rabbithole.
It’s good to remember things, but sometimes an ‘alias’ is more friendly. If you ever forget an alias, like 'what does install do?then typealias install`. We’re encouraged to use pamac, we have pacman too, but I often mistype…
My config .zshrc (it’ll work in .bashrc for a bash terminal) has:
alias purge='pacman -Rnsuv'
alias install='pamac install'
I think the last v was added for verbosity, I forget now 
The main issues we have installing/removing stuff is with ‘variations on standard behaviour’ - for example, we have a .config folder (which is kind of ‘new’) yet .mozilla still skips that and goes straight to ~/.mozilla and then what’s this other ~/.local folder for? Weird stuff makes no sense to me 
/usr/share is for installed stuff - some themes there but also there’s some plasma stuff in .local too… so add them to your bookmarks, edit and label them…
With Dolphin, it pays to add bookmarks for config folders…
/home/ben/.local/share/plasma
/usr/share/plasma/desktoptheme/
/var/lib/plex/Plex Media Server
Over time it seems more logical. Remember, Windows only ever seems easier to people who used Windows for 50,000 hours and Linux for 500 hours. That’s a little simplistic, but some things take time to sink in.
<RANT
Hmmm yes, today my wife asked me ‘Where has My Computer gone??? Why you hide it?’ and I suddenly realised I had no idea how to answer that question without searching…
(answer - in Windows 10 it’s renamed ‘This PC’ which makes no more sense than the old name; and of course, it’s perfectly visible if you press Meta+E - but she can’t cope unless it has an ICON on the DESKTOP to CLICK).