Welcome @bn2qteewnl! I’ve installed ubuntu focal for android stuff with this method:
You should already have several AppVMs pre-installed by Qubes, such as work and personal. If you don’t, click the Qubes menu (top left of the screen in dom0) and click “Qubes Tools” (IIRC) and then “Create Qubes VM”. The defaults in that menu should be fine (fedora Template, sys-firewall for network).
Whichever VM you’d like to download your rpm file in, start that VM by clicking the Qubes menu again, selecting the VM, and then clicking the browser. The browser should launch in that AppVM. Navigate to whichever ubuntu download page and download your file. I’d recommend using an ubuntu template that’s signed by a Qubes dev, and then verify the integrity of the download with a signed hash file, if available.
Note the absolute path, in your download VM, of your focal.rpm download (i.e. /home/user/Downloads/focal.rpm.
Then open a terminal in dom0 by right clicking the desktop and clicking “Open terminal here” or using the Qubes menu again. In the terminal in dom0, run qvm-run -p <download_vm_name> 'cat /home/user/Downloads/focal.rpm' > ~/focal.rpm. This copies focal.rpm to dom0 from the VM you downloaded it in.
Then, again in a terminal in dom0, run sudo dnf install ~/focal.rpm or perhaps sudo rum install ~/focal.rpm.