What is __MACOSX folder?

Apple provides built-in capability to ZIP files in OS X 10.3 and higher, and these files are the result of Apple storing Resource Forks safe manner. You would never see these files running OS X 10.3 or higher, but since Windows and other operating systems do not understand this special form of Resource Forks they will appear as you see them.

"To answer your final question:

how can mac users avoid this mistake?

macOS users can install a 3rd-party archiving utility like Keka, then tell it to not use Resource Forks, then set it as the default compressor.

How to do this with Keka
Tell Keka to not use Resource Forks

Open Keka without a file (From Launchpad, Spotlight, etc.)
Press ⌘ Cmd+, to open Preferences
Select the Compression tab
Check ""Exclude Mac resource forks (eg: .DS_Store)""

Make Keka the default compressor

In the same Keka Preferences window
Select the General tab
Click ""Set Keka as default compressor/uncompressor"" [sic]"