Google creates the Android operating system, then distributes for free to whoever wants to use it. Many companies will tweak the user interface so that it is more customized to their hardware or customer experience, for instance, Samsung uses an interface called TouchWiz. Other manufactures will tweak the UI so much that it's barely recognizable, for instance, Amazon uses Android as the core for their Fire OS which is on their tablets and television set-top streaming devices. On the other end of this spectrum are the Google branded Nexus devices; tablets and phones that are installed with stock or naked Android.
My personal preference is to just leave it alone and stay as close to stock as possible. The Nexus devices are a joy to use with their intuitive interface as well as consistent user experience.
In addition to changing the user interface, many manufactures also write their own versions of apps that would come with the operating system anyway, such as messaging apps, camera interface, calendars and so on. Sometimes these alternate apps are an improvement, but in my opinion, are usually unnecessary.
HTC has taken an interesting approach with the HTC 10. It doesn't quite have stock Android, but it's not far off. I use the Google Now launcher anyway, so it looks and feels very similar to a native Android experience. HTC does have their own versions of many common apps, but they are optional and doesn't come preinstalled on the phone. No duplicate apps! I think this is a brilliant move. If you have a Google account, (if you are using an Android phone, you probably do), then you can automatically sync most of your data to Google's cloud for free. For instance, instead of including a photo browser from HTC, you get the brilliant Google Photos app. Google Photos gives you the ability to browse all of your photos, whether they are on your phone's main memory, on expandable memory via a micro SD card, or if they are on the Google Photo website. All of your photos are also automatically backed up for free, forever, with free unlimited cloud storage. Why wouldn't you want to use this app?
Likewise, you can sync data across devices and platforms with other free Google apps and services such as Calendar, Keep, and Drive. I can access my calendar from any browser from any computer, and can share it with any of my friends or family. My wife and I maintain separate calendars, but can see and even edit each others if needed. We can make appointments that include the both of us and save it on both of our calendars, and get reminders on our phones and tablets. Shopping lists, recipes, AC filter sizes, and all kinds of miscellaneous data can be kept on virtual Post-It notes with Keep, and like calendar, I can access from any browser from any device and share it with anyone.
Another perfect example of HTC's elimination of app duplication is the absence of HTC Fun Fit. You want a fitness tracker? Google Fit is installed on the phone and works perfect. Your data is backed up to the cloud and ready to be used with your next device.
The suite of Google's apps are so useful and powerful, that it only makes sense to use them instead of a third-party app.
HTC realizes that they have some diehard longtime fans who are used to or prefer their twist on things, so they make it easy to just download and install as little or as much of their apps as you want. To my knowledge, HTC is the only company who have been able to strike a perfect balance to a true Android experience and still have customization available. For instance, if you really miss HTC Fun Fit, you can still install it from the Google Play Store.
Full disclosure: I did install HTC's email app, and really like the interface. It has become my go-to email app.
Also missing is a HTC branded flashlight app, or any flashlight app at all. Instead, just pull down the drawer from the top of the phone to turn your flashlight on or off. It is easily accessible while using almost any app, so it is ultimately much easier to use.
HTC has created an ultra-premium device in the HTC 10. It looks and feels beautiful, and the user experience is only enhance by the level of restraint HTC has shown by resisting the urge to mess with the already great user interface that Android Marshmallow provides.
Disclaimer: The Product Ambassadors are Sprint employees from many different parts of the company that love technology. They volunteer to test out all sorts of Sprint devices and offer opinions freely to the Community. Each Product Ambassador shares their own opinions of these devices, therefore the information in this post does not necessarily reflect the opinions of Sprint. The PA's do not represent the company in an official way, and should not be expected to respond to Community members in an official capacity. #sprintemployee