How to get currency symbol from order object

Currency symbol is not stored in order, so to get it, you need to dig a bit deeper and fetch currency object.
I am assuming you have order stored in $this->order so then we get currency code and with that we load commerce_currency through entity type manager and use getSymbol method to get the symbol.

    $currency_code =  $this->order->getTotalPrice()->getCurrencyCode();
    $currency= \Drupal::entityTypeManager()->getStorage('commerce_currency')->load($currency_code)->getSymbol();