ELI5 Smell and Taste - how does our sense of smell work?

Why do "bad smells" like smoke and rotting food appear to linger longer and are harder to neutralize than "good smells" like flowers or perfume? Why is the smell of garlic so hard to get rid of? Why does airplane food universally taste bad? How do our genetics affect our sense of smell and taste with herbs like cilantro / coriander?
... we explain like I'm five
Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: faerlina, bostwickenator, bigdipper80, darth_mufasa, bobatsfight, darwins_dog, unsubtleflounder, godsenfrik and cyberneticpanda
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