I have installed the Vivaldi browser via its deb-repository. It however does not appear in discover in the list of installed apps. Neither with search, nor when I scroll through the list of installed apps. If I search in the list of available apps, the deb-version does not appear as well. The snap version can be installed. However, if I search e.g. in the old style packagemanage Synaptic, vivaldi is shown as it should as an installed app. Repo: https://repo.vivaldi.com/stable/deb/
This means it lacks AppStream metadata and a .desktop file. The former is required for it to appear in Discover, and the latter is required for it to appear in launcher UIs. This is a bug in the app's packaging; please report the issue to whoever packaged it.