On your Silverblue system you have software coming from several sources, using various technologies.
Applications packaged as Flatpak are fetched from Flatpak remotes and use ostree for storage and ostree or container format for fetching (depending on the remote type).
The system is using ostree via rpm-ostree to update from one version to the next and the image content is fetched from ostree remotes.
With rpm-ostree, you can overlay additional RPM packages on top of your base image and those packages will be fetched from RPM repositories.