> 为啥鸟巢用 bird's nest 而苹果树没有's呢？
> 还有为啥是bird's nest 而不是birds' nest呢？？
> 为啥是apple tree 而不是apples tree呢？
I'm not sure why we don't say birds' (or birds's) nest, even though the nest we're referring to should be for more than one "bird". Just remember it for now.
"Apples tree" is wrong. You always use singular noun as an adjective, like "car mechanic", "computer network" (even though the network connects to many computers).
> 我知道100-meter race这种表达是对的
> 但是有没有 100 meters race这种表达的？！
> 或者是应该100 meters' race?
100-meter race or 100 meters race. Either one is OK. I'm not sure if I've seen 100 meters' race (with an apostrophe). Also see message 8 by yuzhoucn at