I have been searching here, but found no solution of how to install gnome-podcasts. With apt I notice it’s not on debian’s or purism’s repos, do I need to install it via flatpaks?
Yep. The flathub.org web page makes discoverability easy. I usually install from there, and manage flatpaks from the terminal.
Must be the same procedure as with mobian, right? https://wiki.mobian.org/doku.php?id=podcasts&s=podcasts
Yep. That’s how I do it if I know the full package name beforehand.