The macOS update checks are a complex topic. You might think that the macOS version is the only thing that gets updated, but the system update does much more. Behind the scenes, it also updates new malware definitions, certificates, and other technical things that ensure your device runs securely.
Every few weeks or months, a new critical security vulnerability is found. If you don't have the latest patch release of your macOS, hackers can get into your machine much easier.
Note that Apple releases security fixes for several major releases of macOS. This does not mean you have upgrade to Monterey but to the latest release of the major macOS version you're using (Big Sur, Catalina, etc.).
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To configure the updates: