Do I have to join a Kupat Holim immediately when I make Aliyah?

Not immediately, but you must enroll within 3 months of receiving your Israeli ID card (Teudat Zehut). Under Section 6 of the National Health Insurance Law 5754-1994, every permanent resident is obligated to join a Kupat Holim. New Olim who enroll within that window are not subject to a waiting period and can use health services from their first day of membership.