For better integration between 5.2 (symfony based) kernel and legacy (4.x) kernel, injection is used to inject settings, session and current siteaccess from 5.2 into legacy using an event: kernel.event_subscriber

Injected siteaccess

When eZ Publish legacy (4.x kernel and extensions) are running via the new eZ Publish 5.2 kernel, then the 5.2 Siteaccess is injected into it to avoid having to match twice and maintain two set of matching rules.

To configure siteaccess in your eZ Publish 5.2 setup, look to the 5.2 documentation of Siteaccess matching.