The current version of Apple’s Mac software is macOS Big Sur, although some people still use older versions of the software like OS X Lion and Mountain Lion. Until recently you had to pay for those versions and now you can download OS X Lion and Mountain Lion for free.
If you wanted to use the previous versions of the software on say an older device then Apple would charge you $19.99 to to get download codes for OS X 10.7 Lion and OS X 10.8 Mountain Lion, you no longer have to pay the fee.
Apple has now stopped charging those fees and they are now offering free downloads of both software versions. You can see the link to download the software below in the support documents for each version of the software.
Apple is also working on a new version of their Mac software at the moment, macOS Monterey which is currently in beta. This software will bring a wide range of new features to the Mac and we are expecting it to land some time in September along with ISO 15 and watchOS 8.