Trump uses an Apple Iphone with only one app installed in it which is Twitter.
In the early days of presidency Donald Trump used an Android device for tweeting which was very old and unsecured, Some security experts have suggested that Trumps Android phone was very much insecure,Trump was using a Galaxy S3 as per the photos and this model did not have any security update since 2015.
Trump has switched to the Apple Iphone this spring, officials said . Because all the Iphone devices regularly gets it security updates and also a it is better than the older android devices when it comes to security.
And also top White House officials wants to limit the screen time of trump because Trump usually wastes his time watching Television and banging tweets on his Iphone. Trump has been asking for more free time but the officials do everything to keep him busy with his schedule rather than tweeting and watching TV all day. It has worked well overseas so far.
So the current device of Trump is Iphone and only one app Twitter which he is best at.