How to create loop in custom page, and get id from url into this loop?

basically what you want to do is get the id from $_GET['id']

then you can use the WP_Query

<?php
    wp_reset_postdata();
    $args = array(
        'category' => $_GET['id'],
        'order'      => 'DESC',
    );
    $query = new WP_Query($args);
    if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post();
//do your stuff here......................
endwhile; endif;
        ?>

here $_GET['id'] will get all the posts from category

If $_GET['id'] is a post ID and you have to retrive all the posts using that specific posts category then simply get the category id from that post by using

$category_detail=get_the_category( $_GET['id'] );

and then you can use the query like:

 $args = array(
    'category' => $category_detail,
    'order'      => 'DESC',
);
$query = new WP_Query($args);

hope this helps