How do I do exposure on something that can't actually happen?

This is a really common question. A lot of people feel their thoughts or worries wont work with exposure for this exact reason. Exposure is in fact about exposing yourself to the possibility of something, not the thing directly. Remember that phobia is when the fear response is disproportionate to the danger of a situation. So in the case of a fear of a plane crash, it would never make sense to be in a plane crash because the fear response in that case would be proportional. However, if you instead are unable to do the things you want to do in life, travel, see family, go on trips, and your fear is holding you back, then it's disproportionate, and exposure is about processing the fear that comes from the thought of a crash and letting the brain re-wire to eventually not produce such a strong fear response.