With the rollout of next iteration of Google maps, Google has made it possible to view custom maps, which have been created by you using My Maps app or have been shared with you by others, in the main Android Maps app itself.
My Maps allows people to specify routes and points of interest on a map. Merging them to Google Maps was not possible, until now. Unfortunately, you still cannot make changes to custom maps when you view them in the main Android map application.
To view your saved maps, launch Google Maps, tap ‘Your places’ in the menu, and scroll down to find your maps, highlighted with red marker icons.
The service is yet not available for iOS and is expected to show up soon. You can get this latest feature by updating your Google Maps application to its latest version.