How to save dropdown slection by a user in the DB

You can update user meta either through custom code or using a plugin. Advanced custom fields will solve your issue.
Check out the reference links for more info :

https://support.advancedcustomfields.com/forums/topic/how-to-use-acf-to-add-user-profile-meta/
https://www.advancedcustomfields.com/resources/how-to-get-values-from-a-user/

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