The promotion entails signing up for the 3 month trial of Spotify premium for $0.99. New accounts only. You then upgrade to the family plan at no additional cost ( it reduces your 3 month trial in to a 2 month trial). The family plan automatically qualifies you for a google home mini. They send a link to the google store, with your promo code already entered, directly to the email you signed up with. Checkout and boom, free google mini (technical a 99 cent mini).
Well peope have been using multiple email accounts and credit cards to get multiple minis. Once their legitimate cc numbers are exhausted, they resort to using virtual credit card numbers along with new email accounts. Fast forward to 2 weeks later, the minis have arrived and are now flooding offer up.