When I get an update, how do I know it's for Mac OS? Some things seem not to apply for my iMac!

Do you have anything particular in mind? Usually updates ship with changes to all platforms. If there's a specific issue for just some platform, that distribution might have a custom rollout strategy at the time to help, and when there's a new version that rollout ramps up faster — but the issue resolved might be affecting everyone, albeit e.g. disproportionally less severely. Feel free to ask specifically, if there's e.g. something that triggered this question in release notes, it might be easier to answer in its context.