Component (a building block) — not the same thing as a radical or a sound component. · 1strokes
乃乙九乞亿几忆艺
Kangxi radical no.5 · 1strokes — The 214 Kangxi radicals
Filed under it in a dictionary. That is a lookup convention — it does not mean it is the only part the character is built from.
乙九乞也习书买乱