Direct answer
FONASA is the national health insurance fund through which Uruguay funds a significant portion of health coverage for working people and their families. The money comes through contributions from salaries and equivalent income.
This system is not simply free healthcare for everyone. The employee and employer make contributions, and then the person chooses a healthcare provider, such as ASSE or a mutualista, and receives services under this scheme.
For people without income, there is a separate path through the free ASSE category if the family meets the established threshold. Therefore, FONASA is better understood as an insurance mechanism for employed people who pay contributions, and ASSE as a public network with different enrollment options.