system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are downloads and storage mediums so accurate, such as keeping billions of bytes without a shred of noticeable corruption?

I am fascinated by Hard drives, and how it seems most people take them for granted. I have had a 1 terabyte Hard drive, and I have had  it filled with games for about 3 years, and none have corrupted or needed to verify the game files. 
My other question is how do downloads stay so accurate, as I thought while downloading a 200 GB file, with zero corruption? It dosent seem like it verifies the content after it downloads, so how do they avoid corrupt downloads?
gpt
The drives themselves have on-board error checking that can remap bad spots out of "spare" space on the drive.  Your 200GB drive may actually have about 205GB space on it, but that other 5G is reserved for remapping those kind of bad spots (those numbers may not be exactly accurate, just an estimate).

The network transport protocols used to move the data between servers also have error detection and correction in them.  Your game download is made up of billions and billions of data packets getting transported to your computer.  Every one of those data packets is checked for an error via a checksum.  If the checksum fails, then the packet is flagged as bad, discarded, and your network card asks for a retry.

And finally, when you install your game, the installer itself verifies the integrity of the files you've downloaded via a hash or checksum.  This allows you to verify the file wasn't corrupted *and* sometimes it's validated against a cryptographically signed hash to verify that it wasn't tampered with somewhere between point A & B.