What usually breaks after a crash?
your bank account
But seriously, the answer is - Gimbal. That is the most fragile and most expensive part.
The gimbal has few weak mechanical spots which usually get affected, and that protects most of the electronic part.
And on second place - is the landing gear. But that is plastic, cheap and easy to replace.
"but what exactly" - I'm pretty sure would be the next question. So, in the order of force of impact:
- The plastic mounting plate / cross plate / dampening plate on top of gimbal - it is there to break and absorb energy of impact, but that's rarely enough
- FFC of course
- Gimbal-to-OFDM ribbon - it often gets some ripped connections; can be put together without replacing, but the molex plug will no longer hold all the wires when unplugged
- motor shafts - the aluminum gets bent near shafts, making tumbling and wrong movement of arms; if the arms are rickety enough, the gimbal won't stabilize at all
- aluminum arms - the aluminum gets bent near bending points, making the gimbal unable to direct straight; trying to straighten that will cause the arm to break
- aluminum arm limiters - each arm has some kind of mechanical limiter for movement - usually kind of bolt; without it, the arm will move further, putting stress on FFC
- A specific quartz oscillator on Gimbal Top Board - when the drone crashes down in its normal orientation, first the plastic plate breaks, then the ribbon gets ripped, then the lens part reaches the ground and that puts pressure on all arms. The Yaw arm gets pushed towards the electonic board, and there is one component there higher than all the others - the long XLAT. so it gets squished, breaking the crystal inside.
Btw, some companies are selling various "strengthening packs" for gimbals. Primary school physics should be enough to know attaching any more mass will only make things worse on impact. The energy of impact needs to go somewhere, it won't magically dissolve into air. And the higher the mass, the larger the momentum, which when negated at the short period of impact releases larger energy.