It's a little bit of everything.
As the other comment noted, there has been a vast expansion of administrative bullshit cost in the system.
Pharma drug costs have gone up a lot (that's around 10% to 15% of the $1.x trillion cost problem the US has). The US is spending about ~20% of its GDP on healthcare, or roughly twice the OECD average (~70-80% more than countries like France or Belgium). That's up from 12.x% in 1990, and 14% in 2000 (that 6% representing an added $1.x trillion in cost). That 12% figure in 1990, is comparable to what nicer developed nations spend today.
US wages are high, healthcare workers typically earn far above the median (eg US nurses and doctors are very well paid), and we've hired a lot of healthcare workers in the last 20 years. The number of healthcare workers has roughly doubled since 1990 from eight million to around 15-16 million today. A near 100% increase vs a population increase of about 30% or so over that time. That is guaranteed to generate a massive cost spike for that part of the problem, and that cost isn't being distributed across a further 100% larger population base. Healthcare workers have also seen their wage growth exceed the median wage growth, they've mostly done ok. So the net cost per capita for each healthcare worker in the US, has soared.
Insurance companies have done exactly what you'd expect. They push prices as high as they can get away with, adding more cost bloat to the system.
The American diet deteriorated dramatically since ~1980. Obesity particularly soared from around the mid 1980s to 2015. That brought with it a lot of added healthcare costs, around cancer, diabetes, and so on. These people weren't/aren't dying so much as they are requiring expensive routine care for various health consequences of the obesity. Conceptually it's like the battlefield cost of having a vast number of wounded soldiers, rather than those soldiers simply dying instead; in this case the battlefield is the economy or society generally speaking.
And then the US population has aged quite a bit in the last 20-30 years. The boomer generation is going to be extraordinarily expensive from a healthcare standpoint. For now that's getting worse by the year as with most developed nations.