Merge-Csv is a function in the MergeCsv module for PowerShell which is now also here on GitHub.
It works well with PowerShell version 3 and up - and might sort of work with PowerShell version 2. PowerShell version 3 or higher is recommended.
Online blog documentation: https://www.powershelladmin.com/wiki/Merge_CSV_files_or_PSObjects_in_PowerShell
It's also published to the PowerShell Gallery: https://www.powershellgallery.com/packages/MergeCsv/
I will add more information here later, but for now check out the online docs which are quite comprehensive.
Here's a screenshot demonstrating some of the more sophisticated features.
