It was all done in a playful manner. And through the many-many patches, you’ll discover that lots of methods has been used. Sometimes directly on the strings – including bowing, ebowing, percussive striking with various implements, and external object-induced vibrations (e.g., fan-induced resonance).
But then also, sounds were often driven through a guitar pedal board and speakers and whatever we thought would be cool to try. It’s a wide palette of coherent piano experiments.