You have two Subpage Templates, which you can use throughout your site as many times as you'd like.
- Subpage Default - Your default two column layout, which auto-generates a side-navigation on the right
- Subpage Fullwidth - Full width layout, can assign full-width sections
The index.php is your home template and assigned automatically to your home page. This templates operates the same as your "Subpage Fullwidth".
Choosing a Template
Go to Content > Pages > Add a New Page (or select an existing page)
When creating a new page, the "Template" field will have the "Subpage Default" template selected as the default. You can select the dropdown menu to change to the "Subpage Fullwidth" template, if needed.
Help site: How to Add a Page