You can either do this inline by tapping the following icon at the end of your song:
Or you can enter a dedicated "structure edit" mode by tapping this icon:
To copy a part, tap on the part label, then on "Copy or Delete".
To copy a line, tap on the line repeat at the end of the line, then on "Copy or Delete".
To adjust the number of bars of a line you can drag the highlighted icon or tap on it: