Cover with skirt/wing
Here’s what it means
Spreading a garment over someone claims protective covenant responsibility and, in Ruth, expresses a marriage request.
Why the Bible says it this way
An idiom is a familiar phrase whose meaning comes from the whole expression, not from reading each word by itself.
A helpful detail: The garment action may be literal and symbolic. Ruth appeals to Boaz as a redeemer, not for a secret sexual act.