How do iPhones and Macs handle OS storage?
When IOS is updated the old operating system is replaced. the same is true for MacOS. So Tetsujin's comment is correct, if less than helpful ;-). But understand the caveats.

IOS sometimes does not delete old temporary files and the file system fills up with junk that is difficult to delete. If you back-up the phone in question to your computer, erase all content and settings and then restore from the backup just created those temp files will be gone. So trying this might give you some of your "missing" space back.

For MacOS when a new version is installed (EG Yosemite to El Capitan) the installer is usually left on the computer in the /Applications folder. I will usually make a restore USB key with that installer and then delete it as they are often fairly large (5GB or so).

With the Mac temp/cache files may also build up with time and utilities like the (free) Onyx will clean up those files and often free up some space.