Not machined or cast, they are 'built'

FDM
Fused Deposition Modelling
A fine (hot) filament of plastic is extruded under the control of an XYZ table to ‘create’ the part in layers, each layer fusing to the one below.
SLA
Stereo Lithographic Apparatus.
Uses a laser to selectively change the state of a resin from fluid to solid.
Again, this uses the layer principle.
SLS
Selective Laser Sintering.
This uses a bed of fine powdered material and selectively melts the material on each layer to the next layer (etc). Depending on the material selected this can produce a working finished part with good characteristics (including steel)

The beauty of each of the above systems is that parts can be made direct from CAD 3D data.
No 2D drawings required.

Other systems are available.
JSD
LOM
SLM
3D printing (many types).


Paul