Dr. Berry, we’re a great fan of your work! Here’s our special offer to you: https://cal.com/21stcapital.com/introduction-call-bitguide At 21st Capital, we help Bitcoiners like you with securing your wealth. As a short answer , Coinbase Bitcoin is simply an IOU. You’ll lose the coin if they default. We recommend a more secure setting including well-known hardware wallets such as ColdCard, @Blockstream Jade and @BitBox .