Google updates as much of the Android ecosystem as it can. First-party Play Store apps, system webview... if you look at the normally hidden system apps on your phone you'll see that the Android team has "unbundled" many parts of the formerly monolithic system to allow updates to as much of it as possible even if the kernel is marooned at an older version.
Unfortunately, some bugs are in the kernel or drivers, so there's nothing any Android OEM (including Google) can do if their chipset vendor won't do the (admittedly non-revenue-generating) engineering to update that firmware. And eventually the system itself requires newer kernel features, so there's a limit to how far back Google or other OEMs can reasonably backport a newer version of Android.
This is part of why Google's recent phones are based on Google-designed, non-Qualcomm chipsets. It was a truly Herculean effort to scrub the Pixel line of Qualcomm, and especially of Qualcomm's incentives to abandon still-good phone hardware in order to sell more chipsets.
Your PC's OS distribution is nearly totally open-source, and the economic incentives for the Linux ecosystem are completely different from Qualcomm's. That contributes to any given general-purpose computer's longevity if it runs Linux.