Modifying data in existing blocks
Yes, your understanding is correct.  In fact, not only are there multiple copies, there are a huge number of copies: everyone who runs a full node has a complete and validated copy of the entire block chain, which they will share with anybody over the Bitcoin peer-to-peer network on request.

In fact, if your hypothetical attacker starts sending out their corrupted block, most users won't even notice: their software will validate it, notice it's incorrect, figure there is just something wrong with the attacker's data, and request it from some other node instead.