To understand how copper works, you've got to understand the basics of iridescence in bettas. There are basically two types of iridescence-
iridescence is what you see on most wild type and sometimes pet store fish. They usually have one or two rows of scales that show iridescence, and the branching on the tail is sometimes irid. Looks like this-
Due to selective breeding and cross-breeding lines to wild types, the spread iridescence
was created, which means the scales and fins are completely iridescent. Some bettas with spread iridescence have unmasked heads, others have masked heads. (This refers to the spread of the iridescence around the gill-to-mouth region.) Spread iridescent bettas look like this-
Copper bettas are technically co-dominant. Crossing a copper to a non-metallic blue betta will result in metallic blue offspring, and inbreeding the offspring will result in some copper and some metallic blues. The same goes for masking on a copper betta- unmasked x masked = some with no masks, some with partial masks. Inbreeding the offspring will result in better masking.
As far as copper x butterfly, look at the example below-
Copper x Blue Butterfly = Metallic Blue (Copper heterozygous), various butterfly patterns on offspring, some without patterning.
Copper x Offspring above = Some copper, some metallic blues, some with various butterfly patterns, some without.
Butterfly is very difficult to breed consistently as it really changes from fish to fish. You may have a fish in one spawn that has the perfect 50/50 color split, while the rest are 70/30, 60/40, etc.