Two different things, Apple provides a pretty long window of OS updates for each device but doesn't ship updates for older OS versions. For example the 1st gen SE shipped with iOS 9 which has not been getting updates for a few years, but it can run iOS 15 which still seems to be getting security updates.
It probably seems odd when you're coming from a history of devices with a very short support window, but you don't need to worry about years when you only get a security update until you run out of years when you get the OS AND security updates first.
For instance, that original iPhone SE came out in 2016, the same year as the original Pixel phone. The iPhone is still supported by Apple today, while the Pixel phone was dropped from support by Google five years ago.
Google had to come up with a way to backport features to older unsupported versions of the OS because their support window was so abysmally short.
Hopefully, this won't be an issue going forward, with Google promising a comparable support window in the future.
