How is Magento\Checkout\Model\Type\Onepage::saveOrder() implemented and used?

Context: How is \Magento\Quote\Model\QuoteManagement::submit() used?

Implementeation

Details: How is Magento\Checkout\Model\Type\Onepage::_prepareGuestQuote() implemented and used?

The single usage: Magento\Checkout\Controller\Onepage\SaveOrder::execute()

How is \Magento\Checkout\Controller\Onepage\SaveOrder used?