The lack of a default file manager on Android (pure Android) was kind of a bummer, but on Android Marshmallow there is a hidden file manager which can be accessed by doing as follows:
1. Open the Settings application and go to the Storage & USB settings
2. Tap the Explore option which can be found under the internal storage items
3. And boom, you have now access to the Android default file manager
Note that this default file manager on Android Marshmallow doesn't offer so much compared to the popular file managers available in the Play Store, but it really is a great new feature on the Android 6.