Explanation
Oracle Cloud Infrastructure Marketplace is an online store that offers solutions specifically for customers of Oracle Cloud Infrastructure. In the Oracle CloudInfrastructure Marketplace catalog, you can find listings for two types of solutions from Oracle and trusted partners: images and stacks. These listing types include different categories of applications. Also, some listings are free and others require payment.
Images are templates of virtual hard drives that determine the operating system and software to run on an instance. You can deploy image listings on an Oracle Cloud Infrastructure Compute instance. Marketplace also offers stack listings. Stacks represent definitions of groups of Oracle Cloud Infrastructure resources that you can act on as a group. Each stack has a configuration consisting of one or more declarative configuration files. With an image or a stack, you have a customized, more streamlined way of getting started with a publisher's software.

Correct. All charges are based on metered usage and calculated as per Oracle's list price or rate card, whichever is lower.

Reference: https://cloudcheckr.com/cloud-security/shared-responsibility-model/
https://docs.cloud.oracle.com/en-us/iaas/Content/Compute/References/nvmedeviceinformation.htm