KSQL is based on Kafka Streams and allows you to express transformations in the SQL language that get automatically converted to a Kafka Streams program in the backend

Kafka transfers data with zero-copy and sends the raw bytes it receives from the producer straight to the consumer, leveraging the RAM available as page cache

The bootstrap.servers setting is used by clients (producers and consumers) toinitially connectto the Kafka cluster. It does not need to list all brokers-just a few that are up and reachable. The client will then use the cluster metadata to discover the full broker list.
FromKafka Java Client Configuration Docs:
"This is a list of host/port pairs to use for establishing the initial connection to the Kafka cluster. The client will make use of all servers irrespective of which servers are specified here for bootstrapping." Hence,listing multiple brokers provides redundancyand fault tolerance in the event one is down during client startup.
Reference:Apache Kafka Java Client Configuration > bootstrap.servers

With this configuration, a single in-sync replica is still readable, but not writeable if the producer using acks=all

The Schema Registry stores all the schemas in the _schemas Kafka topic