$BTC Bitcoin miner will use his or her computer rigs to validate Alice’s transaction to be added into the ledger. In order to stop a miner from adding any arbitrary transactions, they will need to solve a complex puzzle. Only if the miner is able to solve the puzzle (called the Proof of Work), which happens at random, then he or she is able to add the transactions into the ledger and the record is final.

Since running computer rigs cost money due to capital expenditure, which includes the cost of the rigs and the cost of electricity, miners are rewarded with new supply of bitcoins. This is the monetary system behind Bitcoin, where the fees for validating transactions on the network is paid by the person who wishes to transact (in this case it is Alice).

This makes the Bitcoin ledger resilient against fraud in a trustless manner. While it is resilient, there are still some risks associated with the system such as the 51% attack where by miners control more than 51% of the total computation power and also there can be security risks outside of the control of the Bitcoin protocol.