This article explains how to find alternative routes when using Google Maps on your iPhone. You can search for different routes and view their directions before you start navigating or after you've started.
How to Get Alternate Routes on Google Maps iPhone
When you’re planning to leave and want to see the best route to your destination, Google Maps on iPhone will automatically show you those. But you may want to check to see if there are other routes available first.
Tap the Directions icon and enter your starting and ending locations in the corresponding boxes at the top of Google Maps.
You will see the best route on the map with a solid blue line. If there are additional routes available, you will see them in gray, along with the travel time for each route.