When you upgrade SonarScanner alone, are new rules being implemented? Or is that only when you upgrade SonarQube server?
the server upgrade.
Rules are implemented by plugins in Sonarqube server (delivered by Sonarsource or community plugins or custom rules), the Sonarqube scanner just uses them.
Every scanner has to download all those plugins once and again after every update of Sonarqube.
Rules are organized in quality profiles, they are set globally or at project level in Sonarqube.
That said it’s also possible to import findings from third party tools bringing their own rules like i.e. ESlint
This rules are run within the build by other tools and are independent from a Sonarqube update.
see Importing third-party issues
and this discussion