> The main dropper is tiny—an executable file of just 376 bytes.

How does one write code to be that small?

First, don't write in high level languages. Then, tons of effort. Here's a wonderful writeup of an incredibly packed program:

https://justine.lol/sectorlisp/

I suggest you check out the demoscene if you haven’t. This is “Memories” from the Revision competition: https://youtu.be/Imquk_3oFf4

256 bytes

Imagine inserting a smaller file-less malware into WordPress PHP?