Fedora Silverblue uses flatpaks for most apps by default. The exception is Firefox because the codes haven’t been sandboxed yet, but they are working on it. If you don’t need to play videos in Firefox, there is a flatpak available.
I realize Firefox is probably the biggest thing that really should be sandboxed, its why I haven’t switched to Silverblue yet.
The code for the first is there, its just on other branches, they seem to have a very unusual way of using git.
This first one has a QR code linking to its source: https://github.com/niilopoutanen/rss-feed
Unless I missed it, the second doesn’t claim to be open source. In any case this appears to be its source: https://github.com/maco2ou/finefeed
Interesting, I wonder why Silverblue hasn’t switched yet. Or maybe the documentation is out of date.