How Companion Planting Actually Works
Trap crops work through preference, not poison. A pest drawn to a decoy plant isn't being harmed by it (with a few real exceptions like four o'clocks and Japanese beetles), it's just choosing the more attractive option over your actual crop. Plant the trap crop a short distance away so the pests concentrate there instead of spreading evenly across the bed.
Repellent companions mostly work by masking scent, not by being toxic. Many pests find host plants by smell. Strong-scented companions like alliums, mint, or marigolds can make it harder for a pest to locate its target plant by scent, which reduces but rarely eliminates pressure entirely.
Attractant companions bring in predators, not pests. Small-flowered herbs like dill, fennel, and yarrow draw in ladybugs, lacewings, and parasitic wasps, the natural predators that keep pest populations in check without any spraying or releasing required.
Nitrogen fixation happens underground, not through the harvest. Legumes host bacteria in their root nodules that convert atmospheric nitrogen into a form plants can use, but most of that nitrogen stays locked in the roots until the plant is cut down and left to decompose. Harvesting beans for the pods captures the crop but leaves most of the nitrogen benefit behind in the soil, which is actually the point.