[autoavia]

For mass production moulds are usually made from steel which is CNC machined or/or spark eroded. The video in the link explains the basics.

https://www.youtube.com/watch?v=seZqq1qxW30

Prior to CNC machining most moulds were made by using a pantograph. The pantograph followed the shape of a prototype model to cut out the steel mould. This needed a very skilled operator for the pantograph.

 Cottage industry manufacturers either carve a master by hand (eg George Turner Models) which is used to make moulds for resin castings or print a 3D printed body that the customer has to sand and fill.

 I have heard of people taking a mould from a roughish 3D printed body and casting a body in resin. The resin body is then tidied up and detailed to make a good quality master for a further mould which is used for the production bodies. This method is used because it is easier and faster to tidy up the basic resin casting than it would be to try to clean up the 3D print to a high standard. 

 In the 1/43 static model car world some of the cottage industry manufacturers who use whitemetal castings for their models commission a master in brass and make rubber moulds from the master which have to be vulcanised to withstand the whitemetal casting process. The moulds are remade regularly and over time it is not unknown for the brass master to break under the pressure of the vulcanising process but they can usually be returned to the person who made the prototype in the first place to be repaired 
 
 French company AMR run by the late Andre-Marie Ruf was regarded as one of the best producers of 1/43 white metal kits. AM Ruf carved his masters in wax and used the lost wax process familiar to jewellery makers to produce prototypes in silver which were then used to make rubber moulds.

Far more information than you needed but I hope it gives an insight into all the alternatives.