You can build your own Flatpaks from source, too. In fact that's probably how a lot of development is done these days around GNOME and its wider community when

You can build your own Flatpaks from source, too. In fact that's probably how a lot of development is done these days around GNOME and its wider community when using Gnome Builder. You make a change, and Gnome Builder builds a Flatpak and runs it for you so you can test it.