In the early days of Bitcoin, mining used to be done using the CPU on your computer. Nowadays though as the amount of people mining has increased the difficulty of Bitcoin mining has also increased too. This high difficulty and hash rate has mostly been fueled by the introduction of ASIC mining chips. Bitcoin uses SHA-256 cryptographic hash function to secure the blocks and create the hash for each block. This encryption is what protects the transactions in the block from being altered.
