non-glutinous long grain rice
籼 centers on a specific type of rice: non-glutinous, long-grain rice, distinct from the sticky short-grain varieties.
籼 combines 米 (rice) with 山 (mountain), likely as a phono-semantic compound where 山 hints at the sound and the character refers to a type of rice. The exact historical development is uncertain.
The left side is 米 (rice), and the right side is 山 (mountain). Picture long-grain rice growing on terraced mountain slopes, each grain long and separate like the mountain ridges.
For セン, imagine a thousand (千, せん) grains of long-grain rice lined up on a mountain terrace: 千 -> セン, and each grain is distinct and non-sticky.