I'm not quite sure I understand the question??? The most common reason they blow simply by overheating. Getting too hot causes thermal expansion between the head and the block and crushes the gasket armor. Once crushed it can spill water/coolant in the motor, seize up, lose compression. They can blow with nothing going bad internally, simply running the car too hot due to radiator failure, waterpump failure, t-stat stuck closed, coolant line leak etc. If it's caught early enough and water or coolant doesn't destroy the motor, it's not that bad of a fix. When they blow they leak compression and or coolant.
To fix them you of course need new gaskets, and whatever the cause of the gasket failure was to be fixed also. Be it the water pump, radiator, hoses, etc. Internally, it's whether or not the coolant/oil mixture screwed anything up that it would absolutely "need" to be replaced.
