hide on mobile / responsive?

1. I am using bootstrap https://getbootstrap.com/css/#responsive-utilities, so I am using visible-lg class on the navi, so it will hide anything smaller.
2. Nope.

Put example here Lester

This is the exact code I am using for https://lesterchan.net

<?php if(function_exists('wp_pagenavi')) { wp_pagenavi('<nav class="pagination visible-lg">', '</nav>'); } ?>
	<nav class="hidden-lg">
		<ul class="pager">
			<li class="previous"><?php previous_posts_link('&larr; Previous'); ?></li>
			<li class="next"><?php next_posts_link('Next &rarr;'); ?></li>
		</ul>
	</nav>

How can you find out if your theme uses Bootstrap? Are there alternate ways if I don’t have it?

Sorry I don’t have alternative ways. If you don’t know if your theme use bootstrap, contact your theme author.

Hi, where exactly should I place the code above (assuming our theme uses Bootstrap)?

Depends on how your theme is coded, i place it in index.php, archive.php, tags.php and category.php