I have a BigQuery table 'A' with schema {'UUID': 'String'}. I want to join this table with a ADH(Ads Data Hub) table 'B' having schema {'UUID': 'String', 'USER_ID': 'INT'} on UUID and fetch all user_ids to a new table. 
I am having trouble in joining ADH table with BigQuery table.
Can someone please help me out?
If your table A locates in the same region as your ADH account, you should be able to run cross table queries in ADH.
You cannot query user_id as part of the output by using ADH, it's not allowed due to privacy protection. However, if you had your UUID passed to Google Marketing Platform using floodlight with custom-variable enabled, you can use this UUID as join key to map both tables, and do analysis from there.
How to join BigQuery table with ADH(Ads Data Hub) table