Hi @tobinguyen and welcome to the QuickSight community!
So to make sure I understand correctly, you would like to create a drop down of your country names that filters down a visual based on your ‘users’ dataset?
Currently, QuickSight does not support the ability to sync up 2 datasets in this way. The best option would be a table join.
At AWS, our roadmap is primarily driven by our customers, your feedback helps us build a better service. I have tagged this as a feature request to promote visibility to the support team.