"Why it posts to COGS when it sells?"

Because this is how Inventory works.

"my understand is it posts to COGS when it purchase from vendor"

Not for Inventory. Let's review:

Money is an asset. Stuff on hand over a significant time and of significant value is Asset; you didn't "spend" money, you Invested in this stuff that is still on hand. That is why Inventory is also an asset. Trading money in the bank for inventory means Asset = Asset. Nothing really changed, yet.

"and it posts to income when it sells to customer."

The IRS has a rule and the accountant's refer to it as The Matching Principal.

This means the Expense for what is sold, is not Expense, Until it is sold. This removes the value from inventory (no longer on hand) and posts to COG = Cost of Goods Sold. Now that it sold, we match income to expense and see Profit.

Until it sells, it still is On Hand. Value on hand = Asset.

"So if I have one item has -10 quantity on hand, I need use adjustment to give "0" new quantity and post to COGS acount, right?"

Yes, because you are Missing value. The question is, where did that Purchase get posted, since we forgot to list Inventory was what we purchased? If someone posted to COGS when this was an Acquisition of Product to be held on hand, they overstated Expense. It wasn't Gone, already.

"Another situation we have is, sometime we get some parts from damaged machine, we never bought this parts yet, but it can be re-resale, how can i add them to our QB? use adjustment and posts to COGS?"

Yes, as $0. In this case, unless you Buy something, it goes into stock as $0. That means the $0 in COGS in just a placeholder.

"how about the cost/value?"

Value is not Cost. Cost is what you paid for something.

"or i should use items receive without bill?"

Please; I think this is the Third Time---Stop Trying to bring Vendors into this workflow. There is no Item Receipt, Bill or Check or Credit Card Charge unless you Bought Something.