If I create a user with "Salesforce Platform" License in production Org, and assign permission set to create scratch org, will the user will be able to access "Case" object in the scratch org they are creating ? Another way to put it is that.. does the user license type on the production org dictate what objects I can access on the scratch org ? Thanks for your time and help.
The user license and permissions in the Dev Hub org, provided that they allow the user to create scratch orgs, don't affect the scratch org's single user account. That user is a System Administrator.
Your scratch orgs don't inherit the profiles and permission sets from production, so they always start with a blank slate of access configuration.
The org definition file (where you specify edition and features to enable) can determine whether or not certain objects are available at all in your scratch orgs. For example, enhanced notes aren't available unless specified in the scratch org definition or enabled manually. However, this is at the org feature level rather than the profile or licence level.