I saw this app featured on a YouTube channel and thought it was pretty interesting, especially for GNOME users. However there is a rather weird thing about it: it claims to be using Piped for YouTube but it doesn’t get the “Sign in to confirm that you’re not a bot” error. I guess it uses a different API when Piped gives an error.
the Flatpak version isn’t able to find the playlist/subscription JSON files in the file picker on my system
Random guess: Install flatseal and use it to give permissions to that folder.