You can save an area for use in the Maps app on your iPhone and view it on your paired Apple Watch when you can’t access Wi-Fi or cellular service.
With downloaded maps, you can view information like hours and ratings on place cards, get turn-by-turn directions for driving, walking, cycling, or riding transit, and see your estimated time of arrival.
Offline maps are automatically available on an Apple Watch when your iPhone is on and within Bluetooth range (about 33 feet or 10 meters).
To learn how to download offline maps on your iPhone, go here.
Also note that offline maps are available in select areas. Features vary by country and region. See the iOS and iPadOS Feature Availability website.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)
