Several parts combine into one character.

孫 is a compound ideograph: 子 (child) + 系 (xì, "to connect / strand of silk"). The composite reads as "the connected strand of children" = grandchild. The poetic ingenuity is in the 系 component: the lineage is conceived as a thread of silk, with each generation a strand connecting to the next. Bloodline as silk thread. Mainland China simplified to 孙.

Korean reading "son." 子孫 (jason, descendants — used in family-genealogy and historical contexts: "for ten thousand generations of descendants"), 孫子 (sonja, grandson), 後孫 (huson, descendants), 外孫 (oeson, "outer-grandchild" = grandchild through one's daughter — Korean kinship distinguishes inside/outside lineage), 王孫 (wangson, "royal grandchild" — used poetically). Korean family-tree vocabulary depends on 손.

Mandarin sūn, 1st tone (simplified 孙). 孙子 (sūnzi, grandson — the doubled-down term), 孙女 (sūnnǚ, granddaughter), 子孙 (zǐsūn, descendants), 孙子兵法 (Sūnzǐ Bīngfǎ, "Sun Tzu's Art of War" — the 5th-century BCE military treatise by Master Sun, one of the most famous and influential strategic texts in world history). 孫 is also a major Chinese surname: 孙中山 (Sūn Zhōngshān, Sun Yat-sen, founder of the Republic of China), 孙悟空 (Sūn Wùkōng, Sun Wukong / the Monkey King in Journey to the West).

Japanese on-reading ソン (son) — 子孫 (shison, descendants), 後孫 (kōson, descendants), 王孫 (ōson, royal descendant). Kun-reading まご (mago) is the everyday word: 孫 (mago, grandchild — gender-neutral, applies to both grandsons and granddaughters), 孫子 (magoko, "grandchildren plural"), 初孫 (hatsumago, "first grandchild" — major life milestone in Japanese family culture). Japanese grandparents lavish attention on まご in a way that has its own stereotyped vocabulary in popular media.

Memory aid: child (子) connected by a silken thread (系) — bloodline as continuous filament = grandchild.