‘Understanding the needs and expectations of interested parties’, as ISO 9001 puts it, is a critical early stage in developing your QMS. It does in fact relate to more than just customers, it includes for example shareholders, suppliers, contractors etc. too.
But you’re right, it’s mostly about customers. After all, if you don’t know what they want, how can you build a successful system (or even a successful business)? There’s no silver bullet here, you just need to dig and show you’ve done your research: email surveys, phone surveys, customer feedback, focus groups, face-to-face interviews. They all help build a picture. You may find this blog interesting.