How can a hash have a fixed length? I am very new to cryptography, I apologize if my question is so basic. 

I do not understand how a hash can have a fixed length, because you can have infinite inputs to a hash function. With a fixed length you would have limited possibilities of hash outputs. So a hash can have different messages with the exact same output?
So a hash can have different messages with the exact same output?


Yes; that's true of any function with inputs longer than the output.

What collision resistance of a hash function doesn't mean that there aren't two messages $X$ and $Y$ that hash to the same value; instead, it means that it's hard to find them.