This isn't my field, so I should be clear that I'm making stuff up based on podcasts I've listened to :) The way I understand it is that things like MBS's and CDS's were the low-level details of that specific crisis, which we might not expect to happen again in exactly the same way. But the high-level picture was that the failure of major institutions had ripple effects that both 1) directly caused other institutions to fail and 2) indirectly created a lot of uncertainty about who might fail next. Importantly, (2) can cause more failures, by freezing up the markets that a lot of businesses depend on for everyday transactions. This cycle between (1) and (2) can be a general effect, that we might expect to see again in future crises, regardless of what specific low-level details set it off.

Obviously a lot of that debt couldn't be serviced and a lot of it was extended to people who just couldn't afford it.

The derivatives were a multiplier.

It was caused by banks irresponsibly handing out debt and individual investors betting against that debt.