It takes around 20 minutes to spin up HDInsight clusters. So it is not instant but I wonder if databricks on Azure can do any better given they are running in customer environments so wont have a big pool of preallocated multitenant machines available.
AFAIK it is not multitenant, you get your own machines.
They spin up the VMs with either open source Spark or the Databricks runtime - you get to choose the distribution and version before spinning up the cluster.
