Files for Mac can be run on all versions of OS X. You need to uncompress the 7z archive using the proper software (please use Keka to avoid problems). If the archive contains a DMG, double click it to mount the disk. After that, drag & drop the game icon into Applications (or another folder, Desktop will be fine too). New versions of MacOS block applications from identified developers. If you have this problem, just press Ctrl while clicking the game icon, and select Open. Alternatively you can use the “Open Anyway” button in the General pane of Security & Privacy preferences.
See the Help page for more info.