How does buying work — is there a “buy now”?

No. Everything on the Market is sold by auction: the seller sets a starting price, people bid, and the highest bid when the auction closes wins.

How long an auction runs is fixed per collection rather than chosen by the seller, so the countdown on the item's page is the authoritative answer for that item.

Winning is not the last step — an auction you win leaves the NFT in the auction contract until you claim it, and a bid you lose stays there until you take it back. Both are covered below, and both are the answer to most “where did it go” questions.