For each link, you just have to copy the link and give it a distinct name
Assuming you want to list 3 external links:
$link_0 = "https://www.amazon.com/";
$link_1 = "https://www.blablabla.com/";
$link_2 = "https://www.tada.me/";
then for each link, you write the html
<li><a href="<?php echo $link_0; ?>......</li>
<li><a href="<?php echo $link_1; ?>......</li>
<li><a href="<?php echo $link_2; ?>......</li>
And you use the same logic for pages.
Note that if you add many links, you most probably will have to adjust also the nav CSS, to get at least a toggle menu.
