Are printing techniques trade secrets? I'm wondering what type of printer, foil, cardboard, etc., is required to print holographic cards of the sort you might see in Magic or Yu-Gi-Oh. I can find nothing about this online, and I'm wondering how the companies themselves source the industrial printing equipment.
This is only a partial answer.
The core technology originated out of 3M. I'm not sure this link takes you to the "actual" technology used in the cards, but this is an application of that technology.
3M developed the first holographic driver's licenses etc.
They are not trade secrets, but some technical information may be limited online because these companies only communicate with serious clients.
For metallic effects the printer prints white and 4-color ink on top of a board with foil layer. Areas with white behind the color will be opaque and not have holographic effect. Different holographic effects are created based on the type of foil layer. If you look for "holographic special effect foil" you can see foils come with different patterns and rainbow/color effects.
Here is an example of a company that works with holographic foil. You can see they use a lot of technical jargon and you have to reach out to them to learn the details of their capabilities. http://www.crownrollleaf.com/foils_holographic_patterns.html
Looking at holographic Pokemon cards, they use certain patterned foils (dots or stars) that may be custom made for them by their manufacturer.