Short Answer
Publish your script as an editor add-on or as a Google Workspace add-on. Bear in mind that you could make it private, by selecting unlisted or making it available only for your G Suite / Google Workspace organization.
Explanation
Add-ons were added on 2014. This is better than using a library because there will be nothing shared from your add-on.
Please note that
- it’s not possible to access Google Apps Script server-side code by using Chrome Developer Tools or another similar software.
- there are some restrictions for add-ons, study them carefully as maybe this alternative will not work for you.
- nowadays add-ons require a Google Cloud Project and an oAuth consent screen. If the add-on will be shared with external users it will requires an oAuth verification and an app review.
If publishing as add-on is not an option for you, you should rethink your solution considering the use of a web application, using the Apps Script API or the Google Sheets API.