Usually, we often see the term "bitcoin (address) balance" in market information, however, in the Bitcoin system designed by Satoshi Nakamoto, there is actually no concept of balance.

UTXO Transaction Model

Initially, Satoshi Nakamoto invented the UTXO transaction model and applied it to Bitcoin.

UTXO refers to "Unspent Transaction Output", simply put, each Bitcoin transaction is actually composed of several transaction inputs and outputs. The transaction input is the source of the funds, and the transaction output is the destination of the funds. Each transaction must spend a portion from the transaction input, and this portion becomes the unspent transaction output (UTXO). Each transaction input can be traced back to the previous UTXO, all the way back to the initial mining reward. The Bitcoin transactions created from the mining reward are the first transactions in each block, created by miners, and do not have a previous transaction output.

Explaining Bitcoin Wallet Balance using the UTXO Transaction Model

UTXO is the basic unit of Bitcoin transactions, when making a payment, whole number multiples of UTXO are used, such as 1 UTXO, 3 UTXO, there is no concept of 0.5 UTXO. In other words, once a UTXO is created, it cannot be further divided, it can only be spent as an input for the next transaction, and spending it will generate new UTXO, thus continuously transferring the value of the currency. Therefore, the account balance in a Bitcoin wallet is actually calculated by the wallet scanning the blockchain and aggregating all the UTXOs belonging to the user. Therefore, when we say someone owns 1 bitcoin, it actually means that in the current blockchain record, there are several UTXOs with the recipient address being the person's wallet address, and the sum of these UTXOs is 1 bitcoin.

Why can't hackers directly change their address balance, and instead resort to stealing or ransom to obtain BTC?

Because Bitcoin relies on blockchain technology as its underlying technology for storage, it relies on a shared distributed ledger, which is maintained and synchronized in real time by its participants. In other words, as long as a Bitcoin wallet is installed on a computer, it will download the global Bitcoin transaction records, and each wallet will have the transaction records of any holder and the corresponding wallet keys. If a hacker wants to change their Bitcoin balance, they would need to modify every computer with a Bitcoin wallet installed, which is basically an impossible task. In other words, a hacker can modify the Bitcoin amount on their own computer, but every computer on the network with a Bitcoin wallet has a ledger, and if a hacker initiates an erroneous transaction, the other ledgers will directly reject the erroneous transaction.

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