Normally a kit will be created using styrene ('plastic'), which can vary dependent on who makes the kit. The two halves of the sprue mold are machined from steel and styrene under pressure is pumped into the mold, which when separated ejects the parts sprue.
Some styrene is harder and therefore can be more defined in detail. Also these kits normally have location pegs and holes, so alignment of parts is not a problem. Parts are cemented using styrene cement, which melts and welds surfaces together.
Resin kits are made either as production runs or as short runs and, like sand casting, use master patterns to create the molds (similar to vacuum molded kits) in which the resin is poured. Those companies that retail resin as standard, such as HPH Models, are usually of better quality than those from short runs, which can have more problems, such as warping, small air blow holes and mis-molding (short shot). I have some resin parts which are obviously created in a small one man workshop. Some resin kits have no locating pegs or holes so can prove tricky to align parts and parts need to be cemented using CA adhesive, which bonds surfaces together, as styrene cement has no effect on resin.
The new comer is 3D printing, which although it has come a long way, still has its limitations. Various materials are used but for the parts I'm using, acrylic is the base material.
Expense is one disadvantage and small parts can be brittle (dependent on the material used) and even fine printing still leaves 'layer lines' on the surfaces.
However, printing from CAD type design software can produce small and detailed parts. For example look my earlier shot of the 3D printed engine block compared to the kit supplied part.

As the exposed structure for this model is 3D printed, I thought I'd try the 3D printed engine parts available as well.
So far I'm quite impressed with the level of detail although the surface 'layer lines' are evident on close inspection. They can be sand or scrapped away, but this is not really a practical option when working with small complex parts,

Mike