Several parts combine into one character.

者 has an unclear origin, but it was most likely the original form of 煮 (to boil / cook). It was borrowed early for its sound to mean "the one who, the thing that," becoming a pronoun and suffix, while the original "to boil" split off into 煮 (with 火, fire, added).

Korean hanja-sound "ja": 學者 (hakja, scholar), 記者 (gija, reporter), 讀者 (dokja, reader), 患者 (hwanja, patient), 勝者 (seungja, winner), 使用者 (sayongja, user). "~者 (ja)" is the commonest suffix for forming "one who does ~."

Mandarin: zhě, 3rd tone. 记者 (jìzhě, journalist), 作者 (zuòzhě, author), 读者 (dúzhě, reader), 学者 (xuézhě, scholar), 或者 (huòzhě, or), 前者 (qiánzhě, the former). In modern Chinese "~者" forms nouns, as in 消费者 (consumer).

Japanese: on-reading シャ (sha), kun もの (mono). 学者 (gakusha, scholar), 記者 (kisha, reporter), 医者 (isha, doctor), 若者 (wakamono, young person), 人気者 (ninkimono, a popular person). The kun もの refers familiarly to "one who does ~" — 何者 (nanimono, what sort of person).

Memory aid: a character borrowed early for its sound to mean "one who, that which" — the 者 of 學者 (scholar), 記者 (reporter).