iOS

FWIW, Catalina is already unsupported. Whether you can upgrade macOS depends on the age of your Mac. If Catalina is the last version of macOS your Mac can run, you would have to acquire a newer Mac to upgrade.

Macs run Mac OS. iPhones and iPads run iOS. They are completely different operating systems. When you have an older computer which runs an older operating system it eventually becomes non-supported by Apple. When that happens no more updates will be released.If you can upgrade beyond Catalina, then do so. If not, then you can still use the computer as you have been.