Not by default, but you can have the OS have them spin down after a certain idle period. Doing that too frequently can affect the life time of the drive though. You save maybe 4 Watts per drive by spinning them down.
Personally, I make sure my HDDs (regardless use case) never spin down when idle. The cost in life span isn't worth the electricity bill saved.
Same. Also I’ve had some software error out as it waits for drives to spin up (in an NFS share context).
They are never idle if the NAS is seeding torrents.