The files are saved in .zip format, but the original OS in which these files were created is MacOS.
_' is the 'resource fork' part of a file as used by MacOS. There is one for each data file on a Mac. They hold metadata about the file such as the date it was last opened.
More information about this resource fork file:
https://www.tenforums.com/performance-maintenance/127029-thousands-hidden-files-beginning-_.html
If you're not aware of this distinction, you may accidentally unzip the wrong file and attempt to open it in Photoshop, to which you may see an error like this:
There should be two files in the unzipped contents - one of them should not have a "._" prefix. Please re-download and extract the file. If you are previewing the file before extracting, please choose the file without the preceding "_", and you can throw away the "._" file once you have the correct PSD file extracted.
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