If you are using Postman and then select the “Authorize using browser” option, it shows Callback URL that you need to use →
By using that redirect url, when the authorization flow happens, the browser will pass the authorization_code to Postman and then Postman will use it to get the access token.
Similarly for Google App Script, this seems to be a library for OAuth2 - GitHub - googleworkspace/apps-script-oauth2: An OAuth2 library for Google Apps Script.. It also talks about how to get the redirect uri to register with the OAuth provider - GitHub - googleworkspace/apps-script-oauth2: An OAuth2 library for Google Apps Script.