As far as I know, PAMI is for retired citizens and perhaps retired permanent residents who have paid into the system for many years. It is not something an average expat can sign up for.
It is possible for anyone to access to public system, even without an obra social. That's what I have been doing doing for almost ten years and I very pleased with the results.
I am given priority because of my age and, much to my surprise, also because I don't have an obra social (according to a friend of mine).
It also helps that one of my doctors (a surgeon) has become a friend and got me an appointment for a CAT scan during the height of the plandemic a few days after I sent him a message via WhatsApp.