You have already seen them operate whenever you have met those huge trucks with cars on top running down the highway.
Carriers can be open and enclosed and offer either door to door or terminal to terminal services. They are required to obtain both a USDOT number and an MC number from FMCSA. They are required to carry cargo insurance which offers coverage for your car during transport.
Carriers have a limited number of trucks covering a limited area during a week. When their capacity is filled, they will offer subsequent dates when they will be available for shipping.
Not all carriers are created equal
Just as car shipping rates vary, the quality of services does also. Based on past experiences car shipping brokers select the trustworthy for-hire carriers.