What are all of the URI Schemes Available on macOS
Use lsregister
The command can be found in /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister. I suggest creating a symlink to one of the directories in your PATH for easy access.  You can copy and past the following command for simplicity
% sudo ln -s /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister /usr/local/bin/lsregister 

Now, you can run lsregister from anywhere.
Dump the URL Binding Table
lsregister has a number of database tables that contain various application registration details.  The table that we're interested in is URLSchemeBinding
% lsregister -dump URLSchemeBinding

This will give you the full list from afp:// to x-man-page://.
To find the URL scheme used for the dictionary we can grep for it:
% lsregister -dump URLSchemeBinding | grep &quot;dict*&quot;
dict:               35608 (0x8b18) (0x8b1a)
x-dictionary:       35604 (0x8b14) (0x8b16)