Flathub is the central place for building and hosting Flatpak builds. Go to https://flathub.org/builds/ to see Flathub in action.
Application manifests should go in their own repository in the Flathub organization, named after the application ID.
For example, for gnome-recipes, there is a repository named org.gnome.Recipes which has the org.gnome.Recipes.json file at the toplevel.
Hosted builds should be stable releases, not development snapshots, so please use tarballs or git tags, not just the tip of a branch.
More detailed requirements can be found in the Review Guidelines
To install applications that are hosted on Flathub, use the following:
flatpak remote-add flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub org.gnome.Recipes