It’s a specific part of Zeiss, it’s not like they bet the whole group on it. It’s symbiotic because they help ASML get better machines, which helps ASML get more money, a part of which is funnelled back to its suppliers. In the end it’s a risk assessment: what is the expected value if we get closer to a single partner, compared to trying for a broader market. Also, bear in mind that both sides have to keep the other working: the customer without suppliers is just as dead as the supplier without customers.

So: they depend on each other, and they get more business and make more money by getting closer and have incentives to help the other improve (so it is not an adversarial relationship). Hence, symbiosis.