If the CBP feels the need, they will either read through your mail and other apps manually on the device, or dump the device using something like one of these Cellebrite devices. You can see a CBP documentary where this is used on youtube:
I’ve entered the US dozens if not hundreds of times and nobody has ever asked to look at my phones, but who knows how likely it will be for you. You can of course take precautions such as wiping and reloading your phone, implementing pair-locking on an iphone, etc., but it’s all down to your risk appetite whether you wish to do this.
Credit:stackoverflow.com‘