Booking.com complaint: stealing customer money

In April 2021 I purchased 4 tickets through Booking.com, but not be able to travel due to Covid. Even they are not refundable tickets, after called United Airline issued my credits to be used within one year due to special pandemic situation.
Early of this year, I was trying to use the credit. But United Airline told me that Booking.com applied refund in Dec 2021, and the refund money was credited to credit card does not belong to me.

I called Booking.com and wrote many email to Booking.com, they only told me that the refund money is not enough to cover their cost.

I do not understand why Booking.com asked refund from United Airline, without getting my consent, which caused I lost my credit. Why did they use other people credit card to purchase tickets and get refund to that credit card does not belong to me?

Booking.com may steal money from it’s customer: for all none-refundable airline tickets, if Booking.com customer fail to take the flight, Booking.com will ask for tax refund which customer paid, without get any consent of customer and keep the money for Booking.com own profit