From my understanding (which may be entirely wrong but it's not like that stops people on the internet from writing things as if they're correct in all knowledg

From my understanding (which may be entirely wrong but it's not like that stops people on the internet from writing things as if they're correct in all knowledge), Coinbase itself does not give you the $10,000 when they sell. Rather, they are merely a pretty front end for a standard exchange; probably GDAX, the one they also own and operate. So when you go to their site and tell them to give you $10,000 for your 1 BTC, what they actually do is go to their exchange and create a sell order on your behalf. When that order is fulfilled by someone else buying 1 BTC for $10,000, Coinbase takes a cut and then puts the rest into your account.