Installing software on Qubes OS is identical in most ways to installing software on a normal GNU/Linux distro, but there are a couple of differences. From what you’re saying, It sounds like you aren’t experienced in how to install software normally in Linux so I will give a quick rundown.
You normally install software in different distros by using the package manager for that distro. First, open a Terminal. Then, for Debian, use
sudo apt install <package> and Fedora would be
sudo dnf install <package>. You can use
apt search <search query> and
dnf search <search query> to look for packages as well. This link is a good resource for learning how to use different commands to install packages: Rosetta stone - Void Linux Wiki .
The ways that Qubes OS differs in installing software is that you will normally install the software in the TemplateVM (fedora-33 or debian-10 for example) using the normal commands to install the software. Then shutdown the TemplateVM and start or restart the AppVM that you want to use to open the software. You can add the installed application to the application menu by going to the AppVM’s ‘Qube Settings’, clicking on the ‘Applications’ tab, and moving the application over to the right. Also, note that the recommended way to update the software in the templates is to use the ‘Qubes Update’ tool under System Tools in the application menu or from the icon that appears in the notification area when updates are available (appears as an orange star).
If you are trying to install software that is not available in the package manager, let us know what software you are trying to install.
I hope this helps. Please let us know if you have any further questions or problems.