Create username from email, just using the part before the "@"

username_email.php

<?php
add_filter( 'woocommerce_new_customer_data', 'customer_username_based_on_email', 20, 1 );
function customer_username_based_on_email( $new_customer_data ){

    // get the first billing name
    if(isset($_POST['billing_email'])) $billing_email = $_POST['billing_email'];

    if( ! empty($billing_email) && ! in_array( $_POST['billing_email'] ) ){

        // Replacing 'user_login' in the user data array, before data is inserted
        $arr = explode("@", $billing_email, 2);
        $first_part = $arr[0];
        $new_customer_data['user_login'] = $first_part;
    }
    return $new_customer_data;
}