For those who don't know the trick: ZIP files have their index at the end of the file. So you can add a zip file to anything else and have it unzippable.

This was done because it allows adding files to zip archive files without rewriting the whole file, instead just start writing where the index starts and add an updated index at the end. Please note that over the years there's been multiple methods of doing this, including partial indexes which don't even rewrite the index.

If the file you're adding things to is also tolerant of wrong file sizes and extra data at the end (like JPG and many others), you can just:

cat someJPG.jpg someZIP.zip > wth.jpg

feh wth.jpg => shows image

mv wth.jpg wth.zip; unzip wth.zip => works