I would like to add the point that it's easy to find the SHA256 hash but it's very difficult / impossible to find the original string from the hash. Through this mechanism and the proof of work algorithm makes Blockchain a Blockchain. They are secure cyptographically, Not everyone can add a block (like Git) and the blocks are immutable because if you change a block, it would create a new branch and you would need to mine faster than all the miners on earth and keep on increasing the length of the branch to stop it from getting rejected which is impossible. No one has such computing power.