An interesting video, which although brief, gives a quick view of how the coins are minted.
1.Notice the feeder fingers used to move the planchets between the dies.
2.The planchets are all burnished with what appear to be steel or metallic beads which give the planchets a proof-appearance.
3.The rolling mills rolling out the planchet strip.
Because they're bullion coins, weight, metal alloy purity are of the utmost importance, which is why clips or "underweight" planchets are so rare, and also why incorrect alloy mixes are almost unheard of.