You can create a Plugin for the method SavePaymentInformation() in Magento 2 to add your logic before or after Payment information is saved.
The method will be called after a click on the Place order button from the second step of the Checkout page.
Original Class to create plugin:
Magento\Checkout\Model\PaymentInformationManagement Continue reading “Create a Plugin for Save Payment Information Checkout Magento 2.”