So, by default, the launcher will ONLY see projects in the <Documents>\Unreal Projects\ directory. This is something I find deeply annoying.
Thankfully, there is a way to change it!
On Windows, you can navigate to %LOCALAPPDATA%\EpicGamesLauncher\Saved\Config\Windows\GameUserSettings.ini and open that in a text editor. You’ll see a line in the [Launcher] section like:
CreatedProjectPaths=C:/Users/<you>/Documents/Unreal Projects
If you have things in, say, D:\Source\AwesomeGameStuff, then you can change the file to read:
CreatedProjectPaths=C:/Users/<you>/Documents/Unreal Projects
CreatedProjectPaths=D:/Source/AwesomeGameStuff
Next time you run the launcher, et voila, it will also find any projects in D:\Source\AwesomeGameStuff.
On macOS, the file you want will be ~/Library/Preferences/Unreal Engine/EpicGamesLauncher/Mac/GameUserSettings.ini – the change is otherwise the same.