Thank you for your reply. I tried it in a Zorin 18.1 Core live session and Rocky 9, and the installation of podman and distrobox worked, but most apps which I tried to install showed an Error: Unable to find a match, even when I ran sudo dnf check-update before.
The installation of the app cockpit worked, but it couldn't be exported because of no existing desktop file, and then I tried with foliate and terminator, but both showed unable to find a match.
Hm...I wanted to test to install and export just any app, it doesn't matter which one. Perhaps I first should try a well-known distro as LinuxMint. You have no desktop in distrobox, just the terminal, and the command gnome-software didn't work in Rocky container. There seem to be no graphical apps preinstalled.
I'm surprised at how little storage space it takes up. I didn't think I'd be able to try it out during the live session. I even upgraded Rocky, but it didn't help to find the apps.
Edit: I tried Fedora 44 now, this didn't work and showed, when entering the command "distrobox enter Fedora" at Ensuring user's access: Error: An error occured.
Then I tried with Debian 13, it showed problems with setting the German locale, but the export of an app (terminology) worked and the app was shown in Zorin's start menu then and could be launched via the Debian's distrobox container.
Interesting. I've never used something like this before.
I downloaded and installed the .zip package of BoxBuddy, and after running the installation script I was able to launch the program by clicking on the boxbuddy-rs file in the extracted BoxBuddy folder. It wasn't big.
Edit: I noted that BoxBuddy 2.6.0 only showed older versions of available images. It seems not to be up to date. When I searched for supported guest images at distrobox, there were listed newer ones. I think the images are partially outdated, and some images there didn't work (they are not listed on the distrobox site).
I tried the installation of Rocky 9 this time with BoxBuddy, and it worked, also the installation and export of gedit was successful. But I'm still not sure if I would have to add repositories first to find more apps.