The Text Replacements are stored in
$HOME/Library/Preferences/.GlobalPreferences.plist
$HOME is your home folder. Or /Users/your-short-user-name/Library/Preferences/
Because or the leading period in .GlobalPreferences.plist, the file will not typically be displayed. If using BBEdit, then in the Open dialog box, there is a "Show Options" button. Click that and enable "[X] Show hidden items"
NOTE: The .plist file is not pretty, but if you know you wrote a Text Replacement, but cannot remember the trigger phrase, you can search all the Text Replacements in BBEdit, including words in the replacement text, and from that find what the trigger phrase is.
Again, you cannot directly edit the .GlobalPreferences.plist, as the system ignores your changes. You really have to work through the System Settings -> Keyboard -> Text Replacement interface to change any of the entries.
I have over a hundred Text Replacements. I used them for "Canned" replies to discussions.apple.com forum questions. Things like feedback, Safe mode, out of applications memory, a bunch of Apple Watch "Canned" replies. I also use Text Replacement for "Canned" answers to co-workers posting on Slack.
Because I can have up to 2,000 characters (including spaces, and newlines), I use TextEdit -> Format (menu) -> Make Plain Text, to compose the longer ones, then copy and paste the TextEdit composition into the Text Replacement "With" field. If I need to do additional editing, I open an empty TextEdit plain text window, and use the trigger phrase to insert the current Text Replacement, then make my edits, Select All, Copy, then Paste into the Text Replacement "With" field. Click on another entry in the window, then click "Done".
I also share my Text Replacements via iCloud by sharing my Keychain. My iPhone can then use anything I've composed on my Mac. I have found I CANNOT compose anything really long, especially if I want white space and paragraphs, on my iPhone, as I loose all the white space when I compose it on the iPhone either in the Text Replacement window, or in another app, then paste it. All my nice formatting is lost.