@Meinrad This feature came after the course materials were published, so in the next publishing round it will be added and explained.
For the time being, the feature "Upgrade without a restart" does exactly what it promises.
The "Upgrade without a restart" first creates a replica on the upgraded release, then makes sure the systems are insync, the performs a take-over to the replica.
It's like the near zero downtime upgrade you can perform in an on-premises SAP HANA database with system replication. Or if you would think more in a multitenant setup, a tenant move using tenant replication.