The custodian of the Bitcoin is Prime Trust with Fireblocks security on the back end.
Prime Trust acts as the custodian for GiveBitcoin recipients. They are a regulated and insured technology-driven trust company that also serves as the custodian for U.S. customers of Binance, Bittrex, Huobi, OKex and many other major companies in the space. Each recipient has their own user-segregated account holding their Bitcoin, which is never moved, lent out, or rehypothecated in any way.