updated to big sur and now have continuous pop ups

That certainly may be frustrating, but it has nothing to do with the OS.

Download Malwarebytes for Mac (free version) to see if they can get rid of the adware.

https://support.malwarebytes.com/hc/en-us/articles/360038479294-Download-and-install-Malwarebytes-for-Mac

Follow advise from previous posting.

Question - new or old issue ?

Was this an issue before going to the Apple Store where the update was installed?

This is issue occur only after the update was done ?

If it was not present before the trip to the Apple Store - might go back to the Store to explain how malware got installed one the computer.