How does Obamacare save lives?
It does not, by it's self save lives. The general argument is that, in the long run having health insurance will result in better heath care, and that the life expectancy and quality of "everyone" will increase.
I see people on the news testify that if Obamacare is repealed, then they won't be able to pay for their preexisting condition and will die.
In any large system there will be outliers, but in general this is overly dramatic. For people that are "poor enough" medicaid will provide care. It may not be the most awesome care ever, and in may cases that care may only cover the most cost effective options, but people on medicaid can see the same doctors that people without medicaid can. That said, people on medicaid are not entitled to (though medicaid) the same level of care that a millionaire would be able to afford.
There is however a gap between "poor enough" for medicaid to kick in and "rich enough" to afford any and all medical procedures. If you live in that gap then things become more interesting.
Health insurance is/was supposed to be a risk avoidance strategy. On a personal level your supposed to pay more in to it, then you get out of it. On a group level, your group is supposed to pay more in then the group takes out, allowing for more flexibility and cheaper coverage. On the national level the same is true. There should be more income then there is out go.
Where the real trick comes in is pre-existing conditions. Without pre existing conditions, it's historically cheaper to have no heath insurance and pay out of pocket for your normal medical expenses. Big expenses like surgery or long term care are a risk. You would generally try to avoid that risk with insurance.
However pre-existing conditions were generally not covered by insurances. If they were covered it was in a group plan, or the individual paid a higher premium.
But with all that said, these people testifying are mostly, more then 7 years old. They survived before the ACA so if we were to repeal the ACA they would likely survive again, though they may have to devote more of their assets to medical care.
However, the opponents argue that Medicaid will foot the bill and treatment is never denied, even in the days before Obamacare.
Again, there's no such thing as 100%. But even before the ACA emergency life sustaining medical expenses were not an issue. Every ER, hospital, and (I think even doctors office) was required to give life sustaining care. Now this may not amount to much more then stabilizing the patient. But, for example, a hospital could not discharge a patient if the likely result would be death.
Which is true? Does Obamacare cover more than Medicaid?
These things are not the same thing. Medicaid is, at it's core, a bunch of money to pay for medical expenses for people that can not afford medical care. "Obamacare" is, at it's core a statement that everyone should have insurance, and some tools to help people get that insurance.
For most people "Obamacare" did not reduce the cost of medical care. In fact, for most people it increased the cost. For people with long term medical issues the overall cost of medical care may have been reduced.
For example many people claim that they get their "Obamacare" for $100 a month. But what is really happening is that the other $300 a month is being deducted from their tax returns. If their tax return is less then $300 a month then they get no tax return, but the government (other tax payers) eats the difference.
In addition the cost of health care has "exploded" in recent years, in part due to the cost of administrating health insurance.
However heath insurance in general tends to cover more issues then medicaid in general. Rather that coverage is better or not is a matter of opinion.