Kubuntu is ok for a new person on linux?
Yes
I want to install apps/programms via flatpak, since this looks more easily for a newbie as me and because of the sandbox, which seems is good (?).
you can do that on any distro. Should be easy to install.
Other question I have, is about some apps i want to install if i dont find it or just find it on github or something similar. I find an app that really like but this app dont appear in any store, and this app is only available on github via flatpak and .deb . In this case, since the app is not available on flathub or diskover, or via command lines
Thats why after 3years of debian/kubuntu I switched to arch. But that doesn’t mean anything. Just do kubuntu for now and if you have troube installing something ask the community and they will help you get it installed.
And, btw, if you can give me some tips and advice as linux/kubuntu newbie, it would be amazing for my journey.
The best tip for me was to read: https://wiki.debian.org/DontBreakDebian (Since Kubuntu is Debian based, this still applies)
https://borgwarehouse.com/
this is also nice instead of borgserver. Then you can add new clients via web GUI.