You can easily create a nested structure for your custom pages. So some of your pages can be children of others. This would be an easy way to create subpages.
Custom Block
This custom block is created in Admin Dashboard. You don't need to edit any html / php files to create new blocks. Everything could be easily created in admin panel.
You can create several types of blocks including Smarty, HTML, Plain Text or even PHP. Check Admin Dashboard / Content / Blocks management page.