Where are text replacements stored?
Select all the shortcuts you wish to export and drag them to a location in Finder. This will create a ‘Text Substitutions.plist’.
Move this file to another account/computer, then drag it back on top of the list to import the file.


Full details are provided by Apple in their support document on the matter:


How to export and import text substitutions on your Mac - Apple Support




In direct answer to the question title, the actual location used for storing substitutions is within the UserDictionary.db somewhere within ~/Library/Dictionaries/CoreDataUbiquitySupport/. This is not something you should modify manually. Also - since this data is iCloud backed - some versions and point in time it used an older storage method and API (iCloud Core Data) and now it's using a more modern key value store (CloudKit).


https://daringfireball.net/linked/2017/09/26/stucki-text-replacements
https://www.macstadium.com/blog/science-confirmed-text-replacements-do-not-sync/