Here are examples
Search regex in github that will show packages built from .deb, .rpm,..:
https://github.com/search?q=org%3Aflathub+%22type%3A+extra-data%22+AND+%28path%3A*.yaml+OR+path%3A*.yml%29+-path%3A.github%2F&type=code
https://github.com/search?q=org%3Aflathub+%22%5C%22type%5C%22%3A+%5C%22extra-data%5C%22%22+AND+path%3A*.json+-path%3A.github%2F&type=code
https://github.com/search?q=org%3Aflathub+%22.rpm%22+AND+%28path%3A*.json+OR+path%3A*.yaml+OR+path%3A*.yml%29&type=code
https://github.com/search?q=org%3Aflathub+%22.appimage%22+AND+%28path%3A*.json+OR+path%3A*.yaml+OR+path%3A*.yml%29&type=code
https://github.com/search?q=org%3Aflathub+%22.deb%22+AND+%28path%3A*.json+OR+path%3A*.yaml+OR+path%3A*.yml%29&type=code
https://github.com/search?q=org%3Aflathub+rpm2cpio+AND+%28path%3A*.json+OR+path%3A*.yaml+OR+path%3A*.yml%29+-path%3A.github%2F&type=code
https://github.com/search?q=org%3Aflathub+%28%22buildsystem%3A+simple%22+OR+%22%5C%22buildsystem%5C%22%3A+%5C%22simple%5C%22%22%29+AND+%28%22dpkg+-x%22+OR+rpm2cpio+OR+%22appimage-extract%22+OR+%22ar+x%22%29+AND+%28path%3A*.json+OR+path%3A*.yaml+OR+path%3A*.yml%29+-path%3A.github%2F&type=code
Quick search about Source code VS Binary for some packages:
Note: These are Verified and Free Software on flathub.
Browsers
Binary
https://github.com/flathub/zone.dos.Browser/blob/master/zone.dos.Browser.yaml
https://searchfox.org/firefox-main/source/python/mozbuild/mozbuild/repackaging/flatpak.py
https://github.com/flathub/net.mullvad.MullvadBrowser/blob/master/net.mullvad.MullvadBrowser.yml
https://github.com/flathub/app.zen_browser.zen/blob/master/app.zen_browser.zen.yml
https://github.com/flathub/one.ablaze.floorp/blob/master/one.ablaze.floorp.yml
https://github.com/flathub/org.garudalinux.firedragon/blob/master/org.garudalinux.firedragon.yml
https://github.com/flathub/io.gitlab.librewolf-community/blob/master/io.gitlab.librewolf-community.json
https://github.com/flathub/com.brave.Browser/blob/master/com.brave.Browser.yaml
Source Code
https://github.com/flathub/io.github.ungoogled_software.ungoogled_chromium/blob/master/io.github.ungoogled_software.ungoogled_chromium.yaml
https://github.com/flathub/org.gnome.Epiphany/blob/master/org.gnome.Epiphany.json
https://github.com/flathub/engineer.atlas.Nyxt/blob/master/engineer.atlas.Nyxt.yaml
https://github.com/flathub/org.kde.falkon/blob/master/org.kde.falkon.json
https://github.com/flathub/org.catacombing.kumo/blob/master/org.catacombing.kumo.json
https://github.com/flathub/org.kde.angelfish/blob/master/org.kde.angelfish.json
Email Clients
Binary
https://hg-edge.mozilla.org/releases/comm-esr140/file/34b243658c31506d293b13d67238ccca56c290e0/taskcluster/docker/tb-flatpak/repack.sh
https://github.com/flathub/eu.betterbird.Betterbird/blob/master/eu.betterbird.Betterbird.yml
Source Code
https://github.com/flathub/org.gnome.Evolution/blob/master/org.gnome.Evolution.json
…
Its a mess when flathub doesnt enforce source code only (and verified only which is another issue).
Conclusion:
Unless you use gnome software from flathub, its all over the place.