aromatic herb, fine cloth
荃 centers on an aromatic herb, likely used for its fragrance. The meaning 'fine cloth' may be a separate, less common application, possibly related to a fabric made from or scented with such herbs.
荃 combines the grass radical 艹 with 全, which likely contributes a phonetic element. The character is associated with an aromatic herb, but the exact historical development is uncertain.
The grass radical 艹 sits above 全, like a fragrant herb growing completely intact. Imagine a whole, perfect aromatic herb plant.
For セン, picture a thousand (千, せん) aromatic herbs woven into a fine cloth: 千 -> セン, and the scent fills the fabric.