https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Block/Adminhtml/Order/View.php#L190-L207
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Block/Adminhtml/Order/View.php#L455-L464
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Controller/Adminhtml/Order/ReviewPayment.php#L19-L26
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Controller/Adminhtml/Order/ReviewPayment.php#L38-L41
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Model/Order/Payment.php#L882-L889
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Model/Order/Payment.php#L891-L901
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Sales/Model/Order/Payment.php#L903-L919
See also:
canReviewPayment()
acceptPayment()
denyPayment()