Yeah, that's the Play Store though, not Android itself.

I made some Android apps back in the day, so I know what's happening behind the scenes. Mostly it's that Google updates requirements for the apps. Every once in a while, the requirements go up (e.g. "must be targeted for at least Android Version/API Level X", or "Must follow design guideline Y"). The main point for this is that they don't want abandoned apps cluttering the Play Store.

Old apps tend to not support newer features, e.g. the user being able to allow/deny single permissions. On older Android versions you could either allow all permissions or not use the app, while nowadays you can just pick and choose which permissions to allow.

If you install an older app, it won't support this pick and chose system. Stuff like that.

But all that is just Google/Play Store. If you download the APK from somewhere else (e.g. F-Droid) you can totally run apps from 2010 on a modern phone.

That's probably because the Play Store has policies on what apps need to do to keep being downloadable - if you acquire an APK, there's a really good chance that it might still be installable.