Sitecore has two selections for topologies out of the box: XP Single (XP0) or XP Scaled. See here for details.
When you provision these topologies via market place, in essence you are executing these ARM templates which in my opinion are fairly extensible / modifiable.
The roles you've mentioned (Cortex Processing, Cortex Reporting, MA Ops, MA Reporting) are mandatory roles for Sitecore XP, and cannot be disabled. However they can be combined into a single instance which is what you get on XP Single.
So to answer your question, you have a few options:
- You can provision an XP Single topology and manually clone the "Standalone" role (CM+CD+PRC+REP) and then change the cloned role to be a ContentDelivery role
- You can modify the ARM templates for XP Single and do the same.
- You can provision an XP Scaled topology and manually or via ARM template make them all use the same Hosting Plan. In Azure PaaS you are charged for the Hosting Plan, not for the website instances that run on a hosting plan. Of course this means they will share the resources of the hosting plan.
A note of warning is that there is a small learning curve for ARM templates and it is unfortunately a trial and error as there is no way to fully validate an ARM template other than actually running it.