Traditional 萬 originally pictured a scorpion — early Chinese scribes associated the abundant egg-laying of certain insects with "countless / myriad". The character was then specialized to mean exactly 10,000. Both Japan and Mainland China independently simplified to the same form 万 — a rare convergent simplification. So: 繁體 萬 / 新字体 万 / 简体 万.

Critical numeric system difference: East Asian languages count by powers of 10,000, NOT thousands. Where English uses million (1,000,000) and billion (1,000,000,000), CJK languages use 万 (10,000) and 億 / 亿 (100,000,000). So one million in English = 100万 (hyaku-man, "100 ten-thousands") in Japanese/Chinese. This counting-convention difference is one of the most disorienting things for English speakers learning CJK numbers — and vice versa.

Mandarin: wàn, falling 4th tone (simplified 万). 一万 (yīwàn, 10,000), 万岁 (wànsuì, "ten thousand years!" — the traditional cheer for emperors, modernized to political slogans), 百万 (bǎiwàn, million — "hundred ten-thousands"), 千万 (qiānwàn, ten million OR adverb "by all means"), 万一 (wànyī, "in case / if by chance" — literally "one in ten thousand"), 万事 (wànshì, all matters).

Japanese: TWO on-readings. マン (man) is dominant — 一万 (ichiman, 10,000), 一万円 (ichiman'en, 10,000 yen — the largest common Japanese banknote, with Fukuzawa Yukichi on it), 万年筆 (mannenhitsu, fountain pen — "ten-thousand-year pen"). バン (ban) appears in 万歳 (banzai, "Long live!" / cheer of joy — the Japanese WWII-era cheer that English borrowed). Kun-reading よろず (yorozu) means "all kinds / myriad" in poetic and traditional contexts — 八百万の神 (yaoyorozu no kami, "the eight million gods" of Shinto) describes the countless deities of Japanese folk religion.

Memory aid: a scorpion abstracted into the largest cardinal number — many beyond counting.