All Desktops use Kwin and plasmashell but I suppose it is made differently.
Some apps are convergent, so mobile uses either different apps or convergent ones like Discover.
Bigscreen I dont know, wanted to try it once but the packages where really outdated and I havent built an RPM yet
They all use the same KDE stack, just the shell on top is different. They share as much code as possible.
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.