woocommerce-product-variation-settings.php

<?php
// Add New Variation Settings
add_filter( 'woocommerce_available_variation', 'load_variation_settings_fields' );
/**
 * Add custom fields for variations
 *
*/
function load_variation_settings_fields( $variations ) {
  
  // duplicate the line for each field
$variations['ci_text_field'] = get_post_meta( $variations[ 'variation_id' ], 'ci_text_field', true );
$variations['ci_number_field'] = get_post_meta( $variations[ 'variation_id' ], 'ci_number_field', true );
$variations['ci_textarea'] = get_post_meta( $variations[ 'variation_id' ], 'ci_textarea', true );
$variations['ci_select'] = get_post_meta( $variations[ 'variation_id' ], 'ci_select', true );
$variations['ci_checkbox'] = get_post_meta( $variations[ 'variation_id' ], 'ci_checkbox', true );
  
  
  return $variations;
}