kind of grasses, bean stocks & pods
萁 refers to a type of grass-like plant, specifically the stalks and pods left after beans are harvested. It centers on the dry, fibrous remains of bean plants used as roughage or kindling.
萁 combines the grass radical 艹 with 其, which likely provides the sound. The character originally referred to bean stalks and similar grassy plant remains.
The grass radical 艹 sits above 其, like dry bean stalks piling up in a field after harvest. Think of the leftover pods and stems as a kind of grass.
For キ, imagine a key made of twisted bean stalks: key -> キ, and the key unlocks the barn where the beans are stored.