system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does a computer do when it "extracts" a zip or rar file? Where is it getting that extra data from?
gpt
It's extracting the compressed data from within the zip file.  Compression attempts to store the same data in less space through clever math.

As an extremely basic example, say you wanted to compress this string of characters: "aaaaabbbbbcccccdddddeeeee".  Originally, the string is 25 characters long, but you could compress it to 10 by turning it into "a5b5c5d5e5".  A program that knows about this type of compression knows that "a5" decompresses into "aaaaa", but the program that originally created the file doesn't know what these 5s are about.  The computer needs to decompress or extract the string before that program is able to use it.