The short answer to your question regarding flies for salmon is that salmon do not feed when they return to fresh water rivers and streams to spawn. This means that they are not taking or striking at a fly becaue they are hungry and want to eat. Therefore, the flies used for salmon for the most part don't imitate any type of food they eat in the ocean or in fresh water as smolts priot to going out to sea. Thus, the flies for salmon are much more colorful or have spots of bright or flourescent color that are part of the darker, drabber slamon flies.
In very simplified terms, since salmon don't feed upon returning to rivers and streams as adults for spawning, flies used for them need to elicit either an aggression response or kick in a memory of eating an insect or baitfish years ago when they were immature smolts of a few inches in size.
Trout, bass, pike, pickerel, and panfish are very different in this regard because they are actively feeding and looking for things that look like the prevalent insect, baitfish, frog, smaller fish, etc. in the area. That is why we imitate insects and use much smaller flies for trout than salmon.