How to list some posts first in the loop based on post id

If you need to:

• page the query

• retain 12 posts per page instead of “sticking” the desired posts on top of the required 12

• only need to show those posts on the first page

you can try the following

$ids_args = [
    'post_type'      => 'products'
    'posts_per_page' => -1,
    'orderby'        => 'meta_value_num',
    'meta_key'       => '_price',
    'order'          => 'ASC',
    'fields'         => 'ids'
];
$all_posts_ids = get_posts( $ids_args );

// Make sure we have posts before continuing
if ( $all_posts_ids ) {
    // Set all our posts that should move to the front 
    $move_to_front   = [12,13,14,34];
    // Add the array of posts to the front of our $all_posts_ids array
    $post_ids_merged = array_merge( $move_to_front, $all_posts_ids );
    // Make sure that we remove the ID's from their original positions
    $reordered_ids   = array_unique( $post_ids_merged );

    // Now we can run our normal query to display 12 posts per page
    $args = [
        'post_type'      => 'products'
        'posts_per_page' => 12,
        'post__in'       => $reordered_ids,
        'orderby'        => 'post__in',
        'order'          => 'ASC',
        'paged'          => get_query_var( 'paged', 1 ),
    ];
    $loop = new WP_Query( $args ); 
    while( $loop->have_posts() ) {
        $loop->the_post();
            the_content();
    }
    wp_reset_postdata();
}

If you need these posts

• to stick on top of the 12 posts on every page

• in a paged query

you can run two queries as follow

// Set an array of id's to display in front
$move_to_front   = [12,13,14,34];
// Run the query to display the posts you need in front
$args_front = [
    'post_type'      => 'products'
    'posts_per_page' => count( $move_to_front ),
    'post__in'       => $move_to_front,
    'orderby'        => 'meta_value_num',
    'meta_key'       => '_price',
    'order'          => 'ASC',
];
$loop_front = new WP_Query( $args_front );
if( $loop_front->have_posts() ) {
    while( $loop_front->have_posts() ) {
        $loop_front->the_post();
            the_content();
    }
    wp_reset_postdata();
}

// Now we can run our major loop to display the other posts
$args = [
    'post_type'      => 'products'
    'posts_per_page' => 12,
    'post__not_in'   => $move_to_front,
    'orderby'        => 'meta_value_num',
    'meta_key'       => '_price',
    'order'          => 'ASC',
    'paged'          => get_query_var( 'paged', 1 ),
];
$loop = new WP_Query( $args );
if( $loop->have_posts() ) {
    while( $loop->have_posts() ) {
        $loop->the_post();
            the_content();
    }
    wp_reset_postdata();
}

If you only need one page with those posts in front, then the solution by @birgire will do the job just fine