Since iOS 9, you can turn the virtual keyboard into a trackpad on the iPhone. Just press the virtual keyboard harder and a cursor appears over the text, allowing the selection to be made with just one finger. On the iPad you don’t need 3D Touch, just support two fingers on the keyboard to activate the function.
Since iOS 12 there was an improvement in this, allowing the function to be used also in iPhones that do not have 3D Touch.
To use the function, simply hold your finger for 1 second on the space-bar, for the keyboard to become a trackpad.
To select a part of the text, you will need another hand. Like trackpad on iPhone active and the cursor exactly where the selection starts, touch anywhere on the trackpad and slide your finger to the end of the section you want to select. Release your finger and the text is selected.
