shop-stock-product-first-woocommerce.php

<?php
//Change sorting
add_filter( 'woocommerce_get_catalog_ordering_args', 'musilda_show_instock_products_first', 9999, 3 ); 
function musilda_show_instock_products_first( $args, $orderby, $order ) {

    if( $orderby == 'stock_status' ){
  
        $args['orderby']  = 'meta_value';
        $args['order']    = 'ASC';
        $args['meta_key'] = '_stock_status';
      
    }
    
    return $args;

}

//Change default sorting
add_filter( 'woocommerce_default_catalog_orderby', 'musilda_default_stock_order' );
function musilda_default_stock_order(){
  
  return array( 'stock_status', 'ASC' );

}