OS updates happen more often, but Safari’s browser engine (WebKit) gets updated with bug fixes and new features exactly twice per year (in March and September), AFAIK.
New features typically land in March and September, because they are often supported by the underlying OS.
For instance, HTTP/3 support should land in iOS 15 and macOS 12 because it is being added to the operating system frameworks.
Bug fixes can arrive even in maintenance releases, but this is more often seen for security-related fixed (such as remote exploits or side channel attacks)
Safari itself is shipped outside of the read-only container on macOS so that it can be updated independently.
