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The .7z format is the native extension for 7-Zip , an open-source archiver. It is favored in technical circles for several reasons:

to help identify if it's a specific community toolkit? Does 7z compress better than zip? - Microsoft Community Hub

: Attackers sometimes use .7z archives to hide malware. Since many AV tools struggle to scan deep into nested or encrypted .7z files, they are a common vector for phishing.