Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
Yes, you can render react components.
Yes, the component link field supports both internal and external URLs.
No, the entity browser does not work for component defaults as the entities are content, not config.
Yes, this is possible by creating a custom element.
If you want to add custom code to your website you can use Field formatters, Blocks and Custom Acquia Cohesion elements.
No site specific data is stored on the API.
Drupal content field data is NOT transmitted to the API. Only the underlying JSON schema and any information hard coded into templates and styles is transmitted.