Show data from one Custom Post Type in another Custom Post Type

Add a field to your form that allows selection of a machine. You can use get_posts to generate a select element:

$args = [
    'posts_per_page' => -1,
    'post_type' => 'machine'
];
$machines = get_posts( $args );
if( ! empty( $machines ) ){
    echo '<select name="_machine_id">';
    foreach( $machines as $machine ){
        echo '<option value="' . $machine->ID . '" >' . get_the_title( $machine ) . '</option>';
    }
    echo '</select>';
}

Save that value in post meta for the enquiry post under the key _machine_id.

To add a Machine column to the enquiry post type list screen, use the manage_$post_type_posts_columns filter:

function wpd_enquiry_posts_columns( $columns ) {
    $columns['machine'] = 'Machine';
    return $columns;
}
add_filter( 'manage_enquiry_posts_columns', 'wpd_enquiry_posts_columns' );

Next, use the manage_$post_type_posts_custom_column action to output a value in that column for each post:

function wpd_enquiry_column( $column, $post_id ) {
    if ( 'machine' === $column ) {
        // get the machine ID saved in meta
        $machine_id = get_post_meta( $post_id, '_machine_id', true );
        if( $machine_id ){
            // get the machine post
            $machine = get_post( $machine_id );
            if( is_object( $machine ) ){
                echo get_the_title( $machine );
            }
        } else {
            echo 'none';
        }
    }
}
add_action( 'manage_enquiry_posts_custom_column', 'wpd_enquiry_column', 10, 2 );

For the individual enquiry post edit screens, you can add a meta box to display the machine. Here we add a meta box:

function wpd_machine_meta_box() {
    add_meta_box( 'machine-id', 'Machine', 'wpd_display_machine_meta_box', 'enquiry' );
}
add_action( 'add_meta_boxes', 'wpd_machine_meta_box' );

And then the display function for the meta box:

function wpd_display_machine_meta_box( $post ) {
    $machine_id = get_post_meta( $post->ID, '_machine_id', true );
    if( $machine_id ){
        $machine = get_post( $machine_id );
        if( is_object( $machine ) ){
            echo get_the_title( $machine );
        }
    } else {
        echo 'none';
    }
}

Here we have just a simple display of a static value, you can also generate a form field in your meta box, like the first function above, and hook post_save to allow the admin to update the value.