Google Keyword Matching - Confused..

Hi. There are two things that i don't understand concerning the way in which google calculates number of searches that the users do:

1) First off - what is the difference between "phrase matching" and "exact matching". I know that "broad matching" stands for the number of searches of keywords that are base of others (ig. keyword "weight loss" in broad matching would count keywords with wider variations like "weight loss fast" and so on), but how does it work with phrase and exact matching? I read some definitions on Google Help but they're not clear. What they say is that phrase matching "Allows your ad to show for searches that match the exact phrase " meanwhile the exact matching "Allows your ad to show for searches that match the exact phrase exclusively" - i have no idea what's the difference between those two.

2) Now a little different question. If i let's say, try to rank for keyword "how to lose weight really fast". And let's say i include that phrase 3 times in my article. In that circumstance - how many keywords the googlebot would see? Only the whole phrase, which is "how to lose weight really fast", or meybe would count "lose weight" ; "how to lose weight" and "lose weight really fast" as additional keywords? How does it work?

Lastly: when i pick a domain, for example 'bestwaytobuildmuscle.com' , if i get instead 'bestwaytobuildmuscles.com', for google i would rank for 1 keyword (...muscles) or for both muscle and muscles?

PS
sry for my english.

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EDIT
Sorry i have one more question, unrelated to the topic but it's not worth creating a new one: if i have a website, that i created in recent past that is poorly search engine optimized, and i go now and tweak that page to make it more search engine friendly, will Googlebot spot that? Does the googlebot (if this is known) has a cached version of my website and makes a comparision to see the changes, and it says like: " hm, this guy increased his some_keyword density of 2,3%, changed the urls of his pages to match keywords better, deleted some duplicate content and (...) " ? Because i recently learned some tricks to help on-site seo, but i don't know if it won't hurt my website even more if i now apply them.

THANKS