Hello.
The Business Small and higher versions of the Booking Calendar is support Stripe, PayPal (Standard and PayPal Pro Hosted Solution (please note, PayPal PayPal Pro Hosted Solution its does not the same as PayPal Pro)), Authorize.Net, Sage Pay, iDEAL via Sisow and iPay88 payment gateways integration, other payment systems are not supporting yet. You can configure and activate it at the Booking > Settings > Payment page.
Please watch more in this video: https://wpbookingcalendar.com/help/booking-calendar-video-payments/
If you want to make the integration of your payment system by yourself, so you will be able to check how was integrated exist payment system in plugin like Stripe payment gateway. Please check more about this here: https://wpbookingcalendar.com/faq/custom-gateway-in-7-0/
Please note, if you will modify the source code of the Booking Calendar, we will not guaranteed the correct work of plugin and do not support it.