Suppose if you have a tag name “holiday” and you like to display all holiday tag only in your blog. Then you can go to dashboard-> appearance -> menus -> add custom link
View this Screenshot
If you are taking about code customization then while customising the code create a child theme and then customize the code. So that while updating to new version your customized code will not be lost. The content for single.php is displayed from theme folder (interface)-> inc -> structure-> content-extension.php from line 264, you can find this code
if ( ! function_exists( 'interface_theloop_for_single' ) ) :
You can make code customisation over this function.