Question: How can I share common elements across pages?
Question: Can I create a “Master Template” to build other templates off of?
Question: What is the best way to create re-usable template elements - header, footer, navigation?
For elements you would like to share across templates, it is Best Practice to save them as Assets first. Doing so will ensure that every instance of the asset, on all pages and templates using it, will be updated when you make a change to the asset, either through the Finder or Pages and Templates.
Regions cannot be shared across templates, but you can create new templates based off existing ones. To create a “Master Template,” you can build a template with a common layout of regions that you can build off of for future templates. For common styling cues, such as region widths, heights, colors, positions, etc…, it is Best Practice to control them through CSS instead of on the Regions or Template styles directly.
Header, Footer, and Common Nav (ie: Top Nav) items should be built on the template. This will lock the items down to the Template level to prevent them from being modified on an individual page basis. Additionally, saving Headers and Footers as Assets will allow you to edit them in one location to affect site-wide change, preventing there from being any discrepancies for your site branding.