In Mac-OS Catilina, how can I convert the included dictionaries (Shorter Oxford English Dictionary etc.) into a format readable without requiring Dictionary.app?
Apple's Dictionaries are compiled data structures, and Apple publishes no documentation on the format.
This blog provides a detailed description of reverse-engineering Apple's dictionaries.
(I know it's customary to provide an account of the link, but it's a very detailed and complex process.)
There are of course a wide range of existing open source dictionaries.