What's the difference of JCSMPSession.createFlow() and getMessageConsumer()

The basic distinction between the two is where you consume messages from on the broker.

createFlow() creates a FlowReceiver for a guaranteed messaging endpoint - a queue or a topic endpoint.
getMessageConsumer() creates a XMLMessageConsumer - a channel to consume direct messages matching the session's topic subscriptions. Subscriptions are added to the session using the addSubscription(Subscription s) method.
These tutorials explain using publish subscribe and point-to-point pattern with the Java API:

• https://solace.com/samples/solace-samples-java/publish-subscribe/
• https://solace.com/samples/solace-samples-java/persistence-with-queues/