The vast majority of smartphones in the world ship with some version of Google’s Android operating system. And most of them are only supported by their manufacturers for a few years. Have a phone that’s 3-4 years old? Then you’re probably not getting any Android updates anymore. No more security patches. No new features.
Of course, some folks can run custom ROMs such as LineageOS, which lets you install updates indefinitely… but want to break out of Android altogether? There are a handful of other GNU/Linux-based operating systems including Ubuntu Touch, postmarketOS, and Maemo Leste that are designed to, among other things, help give your phone a longer lifespan.