Asymmetric encryption principles, digital signatures, one way hashing, message authentication codes, key derivation

$1)$ Say Bob wants to use his RSA digital signature to sign a $1$GB file. Why is it better to sign a hash rather than signing the entire file?

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$2)$ What is a message digest?

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$3)$ Suppose you use a hash function to generate a $256$ bit hash of a $1$GB file. Using your intuition how can you be certain that an adversary given only the hash value can not reverse the hash function to recover the $1$GB file?

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$4)$ Using the example from problem $3,$ again using your intuition explain whether you believe it is possible that any $256$ bit hash function might given the same hash output for different input messages?

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$5)$ Password systems typically store a hash value of your password along with a random $$salt$$ value that is added to your password when hashing. Why is the salt value needed?

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$6)$ Assume you have a $$hash$$ function where it simply removes vowels from a word. So for example hash$($hello$)$ hll$.$

$1.$ Give an example of a $1$st pre$-$image attack for h $=$hm$($give at least $2$ words that could hash to $$hm$)$

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$2.$ Give $2$ examples of a $2$nd pre$-$image attack for the word $$fluid$.$

$3.$ Give $2$ examples of a collision attack $(2$ sets of words that hash to the same value$).$