Anyone who has a CURP, meaning all Temporary and Permanent Residents, has healthcare. It is called the Institute of Health for Welfare (INSABI).
There is no fee. There is no sign-up. One needs a CURP page printout (not a MX state driver’s license or something showing your CURP number) and photo ID, preferably a Temporary or Permanent Resident visa. A citizen would normally provide an INE / IFE card. There is no membership; there is no age limit. You simply attend to what was a Seguro Popular facility for health care with the two documents I noted.
You may print out your CURP anytime.
Public healthcare at INSABI and IMSS hospitals varies widely. One program is not necessarily better than the other. INSABI in Mexico City is often very good and in some places much less so. In some cities such as San Miguel de Allende, INSABI has a large modern clinic and a hospital whereas IMSS has no hospital and a dated clinic.
Doctors who work with these programs often have a private practice, so to suggest the doctors are of lesser calibre is not true. Doctors who work in private care or at public government facilities do so for low-cost housing assistance, pensions and in some cases to provide care to those who may not otherwise afford their expertise. For many expats, the best option is INSABI which has no fee, supplemented with private insurance.