Please explain custom configuration policy oma-uri settings.

Hy,

For anything that don't work submit an Issue on Microsoft Side or Open a Case.

The docs are not so bad if you find what you need. 

https://learn.microsoft.com/en-us/windows/client-management/mdm/policy-configuration-service-provider

Yes you need to ad ADMX for specific and Custom Settings based on your needs.

Things are failing because are not working , sorry .... :)

Let me try to explain, maybe  this will help you also:

Basic Concept ADMX

An ADMX file is an XML-based administrative template used by Microsoft Windows to define and manage Group Policy settings for computers and users within a Tenant.

What Are ADMX Files Used For?

• Defining Group Policy Settings
• Centralized Policy Management
• Multilingual Support: ADMX files are language-neutral. They work in conjunction with language-specific ADML files to display policy descriptions in different languages, supporting global administrative environments.
• Registry Configuration: Each policy defined in an ADMX file corresponds to specific changes in the Windows Registry, allowing precise control over system and user settings.
• Extensibility: Microsoft provides default ADMX files for Windows and its components, but organizations can also create custom ADMX files to manage settings for third-party applications or custom requirements

How Are ADMX Files Used?

Editing Policies: When an administrator opens the Group Policy Editor, it reads the ADMX files (typically stored in the C:\Windows\PolicyDefinitions folder or a central store on domain controllers) to display available settings.

Deploying Policies: Once a policy is configured, the settings are stored in the Group Policy Object (GPO), and the corresponding registry changes are applied to targeted computers or users.

Central Store: OnPrem -  ADMX files can be placed in a central store (\\domain\SYSVOL\domain\Policies\PolicyDefinitions) so all domain controllers and administrators use a consistent set of templates, Old style :) , new style CSP :)

Ok let´s resume:

1. Configuration Service Provider (CSP) is is a component of the Windows operating system that exposes a standardized interface for configuring device settings.

Delivery mechanism: Settings are typically delivered via OMA-URI (Open Mobile Alliance Uniform Resource Identifier) profiles through MDM platforms.

2. Client Side Extension (CSE) this is the Old Style OnPrem with AD, basically a specialized DLL component on Windows clients that processes and applies Group Policy Objects (GPOs) received from domain controllers.

Delivery mechanism: Settings are delivered via GPOs from domain controllers and applied by the corresponding CSE on the client device.

Don´t try to ingest all the informations :)

Good luck!