The files are a proprietary way for MacOS to store file information (colour labels, time machine information, etc.). When I open properties on Windows, these files are not set to hidden. The following link has a lot of interesting information and links about this.
I get them whenever someone with an apple device sends a ZIP with files, or when I open a shared folder that apple users can access as well. In MacOS Finder these files are hidden. In Unix/Windows they are not. It literally doubles the amount of files in every folder.
The files are always the same:
- In every folder: "._.DS_Store"
- For every subfolder: "._{foldername}"
- For every file in a folder: "._{filename including extension}"