Several parts combine into one character.

Compound ideograph: 冂 (a covering / canopy) above 口 (mouth). The encoded scene: "people speaking from under the same canopy" → "the same / together". Alternative reading: "people sharing the same food vessel" → "together / sharing". Either way, the metaphor is "shared space yields shared identity". Identical across 繁體 / 新字体 / 简体.

同 anchors a wide vocabulary of "shared / same" across CJK languages: 同時 / 同时 (simultaneous), 同感 (sympathy / agreement), 共同 (joint / mutual), 同窓 / 同窗 (classmate — already discussed in 窓 entry), 同年 (same year), 同郷 (same hometown — important social bonding category in Japan and Korea).

Mandarin: tóng, rising 2nd tone. 同 (tóng, same / together), 同时 (tóngshí, simultaneously), 同样 (tóngyàng, similarly), 同学 (tóngxué, classmate / fellow student — extremely high-frequency school vocabulary), 同事 (tóngshì, colleague — workplace vocabulary), 共同 (gòngtóng, common / joint), 不同 (bùtóng, different), 相同 (xiāngtóng, identical), 同情 (tóngqíng, sympathy).

Japanese: on-reading ドウ (dō) for compounds — 同時 (dōji, simultaneous), 同感 (dōkan, "I agree / sympathy"), 同意 (dōi, agreement / consent), 共同 (kyōdō, joint / cooperation), 同級生 (dōkyūsei, classmate), 同僚 (dōryō, colleague), 同情 (dōjō, sympathy), 同性 (dōsei, same gender). Kun-reading おな.じ (ona.ji, same) is among the highest-frequency Japanese adjectives — 同じ (onaji) means "the same" and pairs constantly with comparison vocabulary: 同じ年 (onajitoshi, same age), 同じ国 (onaji kuni, same country).

The Japanese 同 anchors collective identity vocabulary that's culturally important — your 同期 (dōki, same-cohort colleagues from the year you were hired) often forms a lifelong professional friendship group.

Memory aid: shared canopy + shared mouth — speaking from the same covering = same identity.