The key concept of additive manufacturing is really quite simple a 3 dimensional model is created using CAD software by adding material to a build surface in subsequent layers.
It doesn’t matter how nice of a machine or what style or how fast it is if you don’t have a CAD model of your object. The typical formant is a .stl file extension and it stands for stero lithography which was the first type of additive manufacturing invented. The stl file is quite basic containing no information about color or material. It is purely a cloud of flat triangles that closely approximate the CAD model it is not an exact model but is can exceed the resolution of most machines with a few parameters. The next type of file is a .amf file and it stands for Additive Manufacturing File. This type of file is newer and contains more info than just the geometric data. It also contains the color of the model, the volume, and most importantly the triangle making up the mesh of the model are curved meaning that they can match the model much more accurately with fewer triangles and thus fewer points.
The types of way that the material can be added in layers is as follows: material extrusion, material jetting, binder jetting, sheet lamination, laser curing, and laser sintering. See https://www.utwente.nl/ctw/opm/research/design_engineering/rm/additive%20manufacturing/overview-of-additive-manufacturing-processes/ for detailed info above. This article will focus with material extrusion or FDM type of additive manufacturing bacuse this is by far the most common type available.
In FDM the raw material is almost always a thermoplastic filament of either 1.75mm or 3mm that has been wound onto a spool. The most common thermoplastics are PLA, ABS, Nylon, PETG, HIPS, and PC. This filament is fed through and extruder to a nozzle called a hotend. This hotend heats the thermoplastic above it’s glass transition temperature until it is liquid enough to flow through the nozzle. This is then deposited onto either a build surface or a previous layer of the part where it cools and the process repeats again. The key principle occurring here is conservation of volume. What goes into the nozzle must leave the nozzle. The cross sectional area of the filament times the length pushed through the extruder must equal the nozzle diameter times the length of heated extrusion leaving the hotend. The second key principle is that that as the plastic leaves the hotend the plastic starts to solidify. This is incredibly important.