Let's start simple. Your bank account. Your bank account is money that the bank is holding onto for you. The bank has a major incentive to think the number is correct.

They have several safeguards in place. Including multiple redundant systems. Each system keeps all data encrypted, and has a running status list for transactions. Each system also has a hardware security module (HSM) that handles all the encryption and decryption to make keys hard to recover. The HSM has some extremely sophisticated security.

From there the redundant systems keep an eye on each other, always looking to detect tampering in case the HSM somehow can't detect it.

Each of these is heavily backed up, required at least once a day, typically hourly.

So every bank has this. A large bank may have 5000 such redundant systems, along with additional backups. The systems themselves have humans watching them at all times.

So that is how your bank knows exactly how much of your money they have, how much you owe them, etc.

Now every bank is connected to the banking network (runs over the internet, heavily encrypted). Each bank on the network tracks statistic points of every friend bank. So every bank detects tampering with its friends.

The system is not perfect. To provide an additional layer everything is audited, someone actually goes through every even mildly suspicious change to verify it.

So if you were to get inside what could you do?

With minimal suspicion you could move money within a single bank. In particular the contact information on an account has much lower security (only 5x redundancy in most cases). Assuming you can modify all redundant sources quickly enough, you could alter the contact information just before backup, change it back immediately afterwards, audit should accept it. Now you only have to repeat that every single hour every single day, without being caught, for at least 90 days. Now trigger a fault in enough HSM modules to trigger manual backup (75%). The restore would then likely give you that account.