Once order is placed by the customer it goes to load boards - web sites that list available loads for trucking companies to book.
Most popular boards for cars are Central Dispatch and Super Dispatch. Dispatchers of trucking companies bid or book online or by phone.
Broker checks if annual contract with this carrier is in place, signs new one if needed, verifies insurance, reviews etc and confirms booking.
Then carrier moves the load and either gets cash on delivery or gets money from broker or factoring company if it was a credit card order.
Plus there are scenarios with cancellations, receiver not available, non-payments, claims, truck break downs, credit card disputes etc, but those are rare.