Custom registration & profile fields in WordPress without plugin [Only theme template code]

functions.php

<?php

// Shown only one field example, add more fields as needed

/**
* If you have WooCommerce plugin installed then use the following to display fields on the registration form
* If not then check actions/hooks/filters used on your registration page
* To display registration form fields
*/
function iws_extra_register_fields() {
?>
  <p class="form-row form-row-first">
    <label for="reg_billing_company_name" class="font-weight-medium mb-1"><?php _e('Company name', 'theme-name'); ?>&nbsp;<span class="required" aria-hidden="true">*</span><span class="screen-reader-text">Required</span></label>
    <input type="text" class="input-text" name="billing_company" id="reg_billing_company_name" value="<?php if (!empty($_POST['billing_company'])) esc_attr_e($_POST['billing_company']); ?>" required />
  </p>
<?php
}
add_action('woocommerce_register_form_start', 'iws_extra_register_fields');

/**
* Use following if you have Woocommerce installed
* Otherwise use registration_errors filter shown below
* To validate registration form fields
*/
function iws_validate_extra_register_fields($username, $email, $validation_errors) {
  if (isset($_POST['billing_company']) && empty($_POST['billing_company'])) {
    $validation_errors->add('billing_company_error', __('Company name is required!', 'theme-name'));
  }
  return $validation_errors;
}
add_action('woocommerce_register_post', 'iws_validate_extra_register_fields', 10, 3);

/**
* Use the following if you don't have woocommerce
* To validate registration form fields
*/
function iws_learndash_additional_form_fields_validation($validation_errors, $sanitized_user_login, $user_email) {
  if (isset($_POST['billing_company']) && empty($_POST['billing_company'])) {
		$validation_errors->add('billing_company_error', __('Company name is required!', 'theme-name'));
	}
  return $validation_errors;
}
add_filter( 'registration_errors', 'iws_learndash_additional_form_fields_validation', 10, 3 );

/**
* Use following if you have woocommerce installed
* Otherwise use user_register action shown below
* To save registration form field values
*/
function iws_save_extra_register_fields($customer_id) {
  if (isset($_POST['billing_company'])) {
    update_user_meta($customer_id, 'billing_company', sanitize_text_field($_POST['billing_company']));
  }
}
add_action('woocommerce_created_customer', 'iws_save_extra_register_fields');

/**
* Use the following if you don't have woocommerce
* To save registration form field values
*/
function iws_save_user_registration_meta($user_id) {
  if (isset($_POST['billing_company'])) {
      update_user_meta($user_id, 'billing_company', sanitize_text_field( $_POST['billing_company']));
  }
}
add_action('user_register', 'iws_save_user_registration_meta');

/**
 * Display extra fields on admin edit user profile.
 */
function iws_extra_user_profile_fields($user)
{
?>
  <h3><?php _e("Account Section", "blank"); ?></h3>
  <table class="form-table">
	<tr>
      <th><label for="reg_billing_company"><?php _e("Company Name"); ?></label></th>
      <td>
        <input type="text" name="billing_company" id="reg_billing_company" class="regular-text" value="<?php echo esc_attr(get_the_author_meta('billing_company', $user->ID)); ?>" />
      </td>
    </tr>
    <?php
}
add_action('show_user_profile', 'iws_extra_user_profile_fields');
add_action('edit_user_profile', 'iws_extra_user_profile_fields');

/**
 * Save extra fields when admin updates user information.
 */
function iws_save_extra_user_profile_fields($user_id)
{
  if (current_user_can('edit_user', $user_id)) {
  	update_user_meta($user_id, 'billing_company', $_POST['billing_company']);
  }
  return true;
}
add_action('personal_options_update', 'iws_save_extra_user_profile_fields');
add_action('edit_user_profile_update', 'iws_save_extra_user_profile_fields');
?>