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Because they reward length and keyword density. Pages get longer to satisfy the number, readers get less, and the score improves while the page gets worse.
Start from the cluster the page must answer. List the queries reaching it, mark which are actually addressed, and the remainder — in the searcher's own words — is the brief.
Add what the pages ranking above cover that yours does not. That is a comparison of substance rather than of word count.
No. The length is whatever answers the questions.
Yes — that is what makes the page match.
On the next crawl, against the same cluster.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.