Hello,
There are several ways how to handle the shared vs. unique content:
1. the shared content is placed on the master page and the different content is placed on the separate templates. See:
Master page concept.
2. you use the same template for all documents and manage visibility of web parts for particular documents. See:
Visual inheritance.
3. you use different templates. Some custom web parts display the same content (it is enough to edit the web part and the change will appear on all templates).
4. you can use the same template and the different content is placed in the Editable text web part. The content of Editable text is unique for each document even if they use the same template. In general: the content which you edit on the Page tab or on the Form tab is unique for the document whilst the content which you edit on the Design tab is template-related and shared for all occurences of this template.
Best regards,
Helena Grulichova