Both good questions. Mostly, as the droplets evaporate, they stay airborne longer and eventually stick to something (you or another surface). Drying out completely will often kill the virus. If you reuse a mask that cannot be washed, it may start out contaminated or you might leave it sitting around long enough that the virus is destroyed. I leave mine in the sunlight for a week before reusing.
According to the instructions for surgical masks that meet Part 84 approval requirements, the masks are suppose to be kept out of direct sunlight until use..