Here is a link to the official documentation for creating custom content types and new content. Link: https://developer.salesforce.com/blogs/2019/11/use-the-cms-app-to-create-content.html

As per the documentation you could use the below endpoint to fetch details about an existing content type. However, i was not able to find the {your-contenttype-id}. If you find a way please comment it here

Salesforce CMS (Content Management System) allows you to manage content documents.

A content type allows you to define the fields for a content. For example you could have blog posts, faq, how-to-article etc as content types. The below post describes how to create a new content type since this cannot be created with the Setup customization.

Use the workbench to create a new content type. A new content type allows you to define the fields for your content.

Endpoint:
/services/data/v47.0/tooling/sobjects/ManagedContentType/

Request Body:

Note that the 'isLocalizable' attribute determines if this field can be translated or not.
 


The data type of the field that be created is mentioned here. https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_managedcontenttype.htm (Check the nodeType entry in the table)