The applications you are able to see are stored in /usr/share/applications/
This is an example one that I made for Godot Game engine.
[Desktop Entry]
Name=Godot Engine
GenericName=Godot game engine
Comment=Multi-platform 2D and 3D game engine with a feature-rich editor
Exec=/the/location/of/Godotapp
Icon=godot
Terminal=false
Type=Application
You will save it with .desktop extension and then you’ll be able to see it in the menu.
More info; Gnome desktop files
Usually if you install an application trough Debian/Purism repositories should appear in the menu already