You can use almost* all applications regardless of desktop environment, when you install them they'll just pull in all their needed dependencies by themselves (which won't include the full blown DE but just toolkit libraries and other platform components), be it the standard distro package manager or Flatpak

*almost, because sometimes, but very rarely, they depend on some DE specific feature, maybe some Freedesktop portal or a specific Keyring

You may want to look at

https://apps.kde.org

You have video editors, programs for digital painting, apps that sync your phone with your desktop (any desktop, including Windows and macOS), games, etc.

The vast majority work on all Linux desktops, and many work on other platforms (Windows, Android, macOS, iOS) as well.

Desktop environments are always developed alongside a set of applications, which is what differentiates them from simple desktop shells. These applications aren't exclusive to the desktops they were made for, they're simply designed to fit its aesthetic and design philosophy to offer a more consistent experience. Most desktop environments have a fairly limited set of applications, covering mostly the essentials. The most commonly found types are terminals, file managers, settings, image viewers, media players, text editors, screenshot apps and software centers.

GNOME and KDE are the exceptions, by virtue of being much larger than all other desktops. Both maintain a very extensive set of applications with a wide range of purposes, which tend to be widely used regardless of desktops. GNOME's Evolution and KDE's Kdenlive for instance are among the most popular apps on Linux, since they're among the best apps to use to fulfill their respective purposes (PIM and video editing).

@orbitz @m4 From my experience, most DE specific apps will run under another DE. There are exceptions to this and sometimes the amount of dependencies etc needed makes you feel like you are installing much of the DE. Also, some DEs fundamentally use different schemes for launching and configuring settings which can be incompatible with other DEs. There are workarounds, etc but have their own headaches. Other people may have a different experience.

@orbitz @m4 For example yesterday right after installing a KDE desktop, I have installed& run several gnome applications such as gnome disk utility. I really like that one and actually showed professional Mac developers as an example of a unique/functional GUI. Obviously as a person who really adds a mouse pointer set every week, gnome isn't my favourite desktop however I really understand the concept. That is why I don't add 30 extensions to it.

@orbitz @m4 You don't need to be running the desktop environment to run the app, you only have to have it installed. Like, KDE apps require (some parts of) KDE to be installed, but typically work fine even if you're running Gnome instead of KDE, and vice-versa.

Apps do tend to look and feel a little smoother if you use them in their "native" desktop environment though.

@orbitz @m4
I just install stuff on Kubuntu without knowing for which environment it was created. It's possible that Kubuntu has the necessary Gnome support files to make it all work. I added other app stores too. It all just seems to work without me needing to know how.