Text selection in Final Draft Go works the same way as other iOS word processors.
To select text, double-tap a paragraph. A pair of pins will appear, along with the Cut / Copy menu (If there is text in the buffer -- if you have copied something previously -- there will also be a Paste option).
The pin with the blue ball on top is the starting selection point and the one with the blue ball on the bottom is the ending selection point. Drag one or both pins to select the text you need.
When the text has been selected, you can select Cut or Copy; Paste will replace the highlighted text with what was copied before.
Selected text may instead be reformatted by touching an element icon, deleted with the Backspace key on the keyboard, styled with the paragraph style icon or marked or cleared as revised with the asterisk icon.
To paste somewhere else in the script the text you just copied or cut, touch and hold where you want to insert it and from the menu that comes up, touch Paste.