Three peers each hold their own copy of the same chain. Nobody is in charge – the copies are compared.
Try this
Change a block in Peer A only. Peer A’s chain turns red but B and C are untouched.
Which version would the network trust? The one most peers agree on, so an attacker would have to rewrite most of the copies as well.
Mine the changed blocks in Peer A until everything is green again. Is Peer A now “right”? Compare its hashes with B and C.
Puts everything back as it was.
Based on the blockchain demo by Anders Brownworth (source, MIT licence – see licence). Rebuilt here as a self-contained page; this copy runs entirely in your browser and sends nothing anywhere.