Pictograph: originally a large wooden vessel or bell — "an implement / tool to be used". The character abstracted from "implement" to the verb "to use / employ" itself. Identical across 繁體 / 新字体 / 简体. One of the foundational verbs in CJK languages.

Mandarin: yòng, falling 4th tone. 用 is one of the most-used Mandarin verbs — both as standalone and in compounds: 用 (yòng, to use), 有用 (yǒuyòng, useful), 没用 (méiyòng, useless), 不用 (búyòng, no need), 不用谢 (búyòng xiè, "you're welcome / no need to thank"), 用力 (yònglì, exert force), 用功 (yònggōng, study hard / be diligent), 用心 (yòngxīn, attentive / earnest), 使用 (shǐyòng, formal "to use"), 应用 (yìngyòng, application). The colloquial 不用 + verb covers a vast range of polite refusals: 不用客气 = "no need to be polite".

Japanese: on-reading ヨウ (yō) is dominant — 使用 (shiyō, use), 活用 (katsuyō, utilization), 用意 (yōi, preparation), 用事 (yōji, errand / business — used in daily scheduling: 用事がある = "I have something to do"), 利用 (riyō, use / utilization), 採用 (saiyō, adoption / employment — used for hiring), 不用 (fuyō, unneeded — note different from Mandarin's 不用), 雇用 (koyō, employment). Kun-reading もち.いる (mochi.iru) is the formal verb form — used in academic and literary writing.

The Mandarin 没用 / 没用 (méiyòng, "useless") is one of the most useful daily-life Chinese expressions — applies to broken things, ineffective methods, or self-deprecating humor.

Memory aid: an old vessel — what one uses, abstracted to the verb "use".