WHOLE CHARACTER
笔bǐ
pen; pencil; brush (毛笔, 铅笔); a stroke of writing (笔画); to write (笔者, 代笔); (measure) a sum of money (一笔钱)
笔 is a pen — bamboo and hair together form a traditional writing brush.
ZDIC: 会意, 从竹、从毛 — bamboo and hair, the parts of a writing brush.
BAMBOO TOP (THE BOUND FORM ⺮)
⺮zhú
bamboo (from 竹)
The paired bamboo crown (the bound top of 竹); the written top of bamboo-related characters; not a standalone word.
In 笔: bamboo top (the bound form ⺮) The bound bamboo-top form; it differs from the full character 竹.
HAIR PIECE (毛)
毛máo
hair; fur
A standalone character; named by ZDIC as a piece in 笔 and 表.
In 笔: hair piece (毛) Shown with its documented or clearly limited role.