So I tried out the Steam version of the editor, created a new project. On Windows, it defaulted to create it in my Documents folder: "C:\Users\MY_NAME\Documents\New Game Project". ("MY_NAME" being the name for my computer's account). I closed the editor, then opened it back up and could find it by typing in a part of the name into the search box at the top-right of the Project Manager window.
Also, Godot uses its own unified directory to avoid problems if you move from one system to the next. The directory "res://" is the local directory for your project. As stated above, mine defaulted to the Documents folder on my C drive, though I am unsure of the default locations on macOS and Linux OS's.
If you want to find where exactly your projects are going if you cannot remember, create a new project, then go to the "FileSystem" tab. Right-click on the folder called "res://", then at the bottom of the drop-down menu, click "Open in File Manager". That should take you to the correct location for that new project, and if you have not been changing the locations of all your other projects, the others should be there, too. You can make a shortcut to the location, or write down the file path so you can navigate there again later.
Hopefully this helps!