You can find the OS for your runtime in the system environment information in release notes, for example for Runtime 16.1 it can be found in: https://docs.databricks.com/en/release-notes/runtime/16.1.html#system-environment which is Ubuntu 24.04.1 LTS.
As for pricing you can use the pricing calculator: https://www.databricks.com/product/pricing/product-pricing/instance-types
