I tried searching on Google for something touching on this but I couldn't find anything.

Couldn't an attacker just send along the hash of their modified data to the receiver? I assume that the receiver would just run the hash through the algorithm, and since the hash matches the data (albeit the modified data), the receiver would just go ahead and say "Okay, this data has been unmodified". I suppose that hashing is a lot more complicated than this, but it was just a question that's been in my mind since I started learning about hashing, and I would like to address it.