As described in Chapter 5, Installi

As described in Chapter 5, Installing a Bitcoin Node, after installation, a client will connect to the network and begin requesting each block sequentially from the other nodes on the network. After checking each block against the previous blocks, one by one, the blockchain is replicated and stored locally.The local copy of the blockchain contains every transaction from the beginning of time. This ledger is maintained and used to validate the spending balance of each new transaction. If a transaction is found to be invalid, it is simply ignored and discarded.The second check requires validation of the digital signature. Using cryptographic algorithms, the miner is able to check the signature attached to the transaction to validate the integrity of the transaction. Any modification to the transaction will result in an invalid signature, and thus the miner is able to confirm that the transaction to be processed was the original version sent from the holder of the private key.With a list of valid transactions, the miner assembles a new block and uses it as the base for solving a difficult computing problem.