How do I install one Android phone's apps to another Android phone without rooting? How do I install one Android phone's apps to another Android phone without rooting ? I need to install the Samsung Galaxy S Duos Internal Music Player on a HTC Desire 620G. Please help me, and thanks in advance.
To your problem, you cannot do so, unless you root both your phones. 



Samsung Galaxy S Duos's music player is a System App, if I remember correctly. System Apps can't be copied like normal Application Packages unless you are rooted. You have mentioned that you need a way to do so without rooting, and it is not possible. 
If it is a User App i.e. if you have installed them yourself, then you can just copy the APK file present in the data folder and install them onto another phone. 

Instead of doing so, I would recommend you to use the Shuttle music player. It has many features, and can be used as a replacement for any device's music player. 



If you prefer rooting by any chance, then you can do so (there will be a few bugs, because of porting System Apps from a Samsung TouchWiz UI based ROM to a HTC Sense UI based ROM) by using the Link2SD app and the following procedure : 


Open the Link2SD App
Convert your specific app into a User App
Get the APK file of that app, using an APP backup tool or any similar one which converts installed apps into installable APKs
Copy the APK to your other device (in this case, your HTC Desire 620G
Install it as a System App using the Link2SD App. 
Use it. 




Note that the App might have a possibility of crashing when being converted into a User App, and because of the dependencies of the app on the specific frameworks of the specific devices, it might not work at all, and might even brick your device. Do so with caution.