The bulk is spent on inpatient hospital stays I believe. People like to blame pharma, but hospitals charge absurd prices and many times just make people sicker.
My wife and baby stayed 3 days at the hospital after the c-section. Charge for room and board item on the bill is $30,000 for her and $50,000 for the baby. In total $80,000 for 3 days.
And it was shared room. There was another woman with her baby with them. That single hospital room's rate is $53,000 a day.
All other charges are 10% of room and board.
No idea how insurance affects prices. But here's another anecdotal evidence: When we were poor (minimum wage level), my wife was on Medicaid or Medicare; think we did not pay anything for it. I'd remember having to pay anything over $1,000 back then. Don't have the receipt to actually say how much hospital charged and how much Medicare payed for it. In that case we'd have 2 points to compare.
And that was baby No.3 that part of my life is done :D