On Mac
Follow these steps to open and decompress .7z files on a Mac:
1. First you need to download Unarchiver (it’s free, you can also get it from the Mac App Store)
2. Launch Unarchiver and you’ll see a file association list, tell Unarchiver to associate with .7z files (you can select others if you wish)
3. Once Unarchiver is associated with the .7z you can then double-click any .7z file on your Mac and it will open and uncompress like any other archive format, or you can launch Unarchiver and drag and drop 7z files into the utility
4. Let the decompression of the 7z complete before attempting to open or interact with the extracted files contents
On Windows
Instructions and screenshots here: https://www.wikihow.com/Open-7z-Files
1. Navigate to the 7-Zip website. Because 7z files are compressed archives, you can’t view their contents until the files are extracted with an unzipping program
2. Click “Download” next to the file for your version (32-bit or 64-bit) of Windows.
3. Select a save location (such as Desktop), then click “Save”.
4. Double-click on the 7-Zip .exe file and click “Run”. Follow the prompts to complete the 7-Zip installation.
5. Double-click the 7z file you want to open.[2] The contents of the archive will display in the 7-Zip window.
6. Press Ctrl+A to select all of the files in the archive, then click “Extract”.
7. Click the “…” button to choose a folder to store the files. The folder you select is where the files inside of the 7z archive will be extracted.
8. Click “OK” to unzip the files to the folder. You’ll see a progress bar for the duration of the unzipping process. When the files are unzipped, the progress bar will disappear. At that point, you can double-click the folder to access the files.