magento2.3 – Where do I add this ticket booking step when paying?

I'm implementing a cruise ship booking site on Magento 2.3.
I do not know which modules or files I need to implement steps 3 and 5 below?

(User feeds on the website)

  1. Guest sails through the available cruises. Choose one by adding to the basket. Click on "Order"
  2. The guest gives the names and addresses of the passengers, recorded on the payment page of notices and payments. Clicks "Place order"
  3. The system must connect to the cruise provider via an API and make a reservation "on hold".
  4. In case of a successful "suspended" reservation, the system must process the payment via the payment gateway.
  5. If payment is successful, the system must connect to the cruise provider and request that the "pending" reservation be converted to a "confirmed" booking.

I have the Cruise Provider API (which is in SOAP) – so no problem here.

Which modules should I replace or in which files should I write my code to perform steps 3 and 5?