After registering on Google Pay, you can open the app to find your UPI ID.
Your UPI ID will have either @oksbi, @okhdfcbank, @okicici, or @okaxis at the end.
Log in to the Google Pay app
Click on the Profile button on the right hand side
Click on the Payment methods
select the bank which is registred on Upstox if you have added two or more bank on Google pay
Select the bank which is registred on Upstox if you have added two or more banks on Google pay
Check the UPI ID, under the manage UPI IDs