To download Google Maps directions on your phone, you must download the that contains your entire intended route . Google Maps does not have a single button to "save a route" for offline use; instead, it provides turn-by-turn driving navigation by referencing the map data saved locally on your device. How to Download Offline Maps (Android & iPhone)
The process is nearly identical for both Android and iPhone users: Download areas & navigate offline in Google Maps - Android how to download google map directions on phone