Check if user is logged in when clicking certain links on certain pages

You might start by looking at the is_user_logged_in() pluggable function.

https://developer.wordpress.org/reference/functions/is_user_logged_in/

This is basically two lines:

$user = wp_get_current_user();
return $user->exists();

In your case, wrap the links inside the curly braces of a conditional.

if ( is_user_logged_in() ) {

    // link
}