Fortunately, as treatments have improved, fewer people are being ventilated. Unfortunately, (and you can look up the statistics yourself) if you are ventilated, the outcome isn't that positive.
There's some debate about the average case mortality rate, partly because nobody really knows for sure how many people have been infected. Nevertheless, it's much much higher than average amongst the elderly and in certain groups, including those who are considered clinically extremely vulnerable.
Nobody really knows why some individuals are affected, although it seems some theories have been discounted. It's probably something to do with T cells and ACE2 receptors and the amount of inflammation already existing. People aren't tested for them, so don't really know what's going on inside their body. There's still a lot of research to be done on auto-immune conditions, which aren't that well understood and there could be a link. Who knows?
Even something as common as T2 diabetes isn't properly understood. I developed it when I was 36. I wasn't overweight, ate reasonably healthily and was quite fit. I don't have a family history. My sisters have never developed it, although both are overweight. It's known that there are various risk factors, but there are still people who are atypical. It's now thought that it has something to do with inflammation, but nothing is known for certain. There's an answer somewhere, but it hasn't been found yet and I suspect it's the same for allegedly low risk people and Covid.