>This is not a Xapo wallet anymore, this is a BitGo custodial wallet for Bitstamp's cold storage that was migrated October 2019 and you can trace the coins from there.
A company I know of uses modified laptops across multiple locations and each one has a shard of the key.
Chipsets are removed. Disabling all USB / bluetooth / ethernet / sound / etc. I dont recall the exact details but apparently there is a market for devices like this.
What's the point in using a decentralized exchange medium if you're ultimately storing your money in a centralized exchange and giving them nearly the same level of power over your money as a bank?
Is it possible to securely transfer bitcoin offline?
Like if someone hands you a thumb drive and says “this contains a key for a $1m wallet”, you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?
No, if the transfer is offline, there can be no guarantees that the same person didn't try to double spend by transferring the same funds to someone else (unless you involve a trusted third party).
No, that's not what the question was. The parent poster specifically said "you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?" In your example the account key on the usb stick is not guaranteed to be the only copy, and thus, there is risk of the sender double spending that money.
In other news: An unknown amount of Monero was transfered from an unknown wallet to an unknown wallet.