What is site Icon?
It is small icon on the tab bar of your website
What is title and tagline?
They are name and short description on title of your site.
Favion along with site title and Tagline is found in dashboard>customize>site identity
During development of wordpress you have to hard code for showing these.
First of all lets discuss about the site title and Tagline. You need to put these code wherever you want to display site title and tagline. (usually inside tag.)
echo get_bloginfo(‘title’); for site title and
echo get_bloginfo(‘description’); for Tagline
NOTE: you should keep the tag inside php code <?php ?>
For Site Icon. You need not put any code. The CMS system of wordpress automatically changes the favicon whenever you change the site icon from customizer.