This is outdated documentation made for eZ Publish Platform 5.1. It is being moved into the eZ Publish 5.x documentation, so please go there for most up-to-date documentation.
For better integration between 5.x (symfony based) kernel and legacy (4.x) kernel, injection is used to inject settings, session and current siteaccess from 5.x into legacy using an event: kernel.event_subscriber