No you won't because it's not a transaction on the blockchain. You are buying it from a 3rd party who bought it somehow and their transaction appeared on the blockchain but you can't see it.

They can actually put fake BTC in your account if they please however if BTC goes to $20K and everybody wants to cash out they will be on the hook for the difference. Since they aren't some scammy bucket shop Forex broker they would need to deal with regulators. So most likely they hedge their exposure somehow. Even with the futures markets.