> Market still manages to function, and even without the individual mandate. How exactly?

The ACA tried to address this question with the individual mandate. It also has an enrollment period. You can't sign up on a whim.

Those two features were meant to prevent opportunistic purchases of health plans. Only buying the second you need it, rather than paying in consistently over many years.

It doesn't seem to be working well, because our healthcare market is hardly "functional" from the user's perspective.

I'd say that's because healthcare is something that fundamentally cannot be subject to market forces. Your life is literally priceless, and your health is worth more than you own. It is not a good or a service or a commodity, so it cannot be priced according to basic market theory.

In the US, our healthcare market isn't functioning. In other countries, they've solved for this by eliminating the market aspect, and treating healthcare the same as roads, fire, police, etc.