Several parts combine into one character.

The traditional 寫 was a phonetic-semantic compound: 宀 (the "roof" radical, indicating an enclosed space / house) over 舄 (xì, a magpie — providing the sound). The original meaning was "to copy / transfer text inside the house" — the work of a scribe in a writing chamber, transcribing a text by hand. Japan and Mainland China both performed one of the most aggressive simplifications in the entire reform: the entire complex 舄 inside the roof was cut away and replaced with a small mark, leaving 写 — a character that bears almost no visual resemblance to its origin. Korea preserves the traditional 寫 in formal writing, which Korean learners often find startlingly different from the Japanese/mainland 写.

The character carries three semantically related senses: copy / transcribe / project an image. The "photo" meaning is the most striking modern extension — when photography arrived in Asia in the 19th century, 写真 was the coined compound, literally "to copy the real." Photography was understood as the ultimate form of accurate transcription.

Korean reading "sa." 寫眞 (sajin, photograph), 複寫 (boksa, copy / duplication), 寫實 (sasil, realism in art / depiction of facts), 書寫 (seosa, writing / transcription), 寫經 (sagyeong, "to copy a sutra by hand" — a Buddhist meditative practice still observed in Korean and Japanese temples).

Mandarin xiě, 3rd tone. Mainland Chinese pivoted 写 toward "write" as its primary meaning, partially eclipsing the older "copy / transcribe" senses. 写字 (xiězì, to write characters), 写作 (xiězuò, to compose / authorship), 写信 (xiěxìn, to write a letter), 写日记 (xiě rìjì, to keep a diary). The "photograph" sense survives in 拍照 (pāizhào) using a different character, though 写真 (xiězhēn) does appear in Chinese, especially in art-photography contexts borrowed from Japanese.

Japanese on-reading シャ (sha) — 写真 (shashin, photograph — Japanese coined this Sino-Japanese compound and exported it back to China and Korea), 複写 (fukusha, duplication), 写生 (shasei, sketching from life — drawing what you see). Kun-readings: 写す (utsusu, to copy / project / photograph), 写る (utsuru, to be reflected / to appear in a photograph). The 写真 etymology is identical across all three CJK languages: "to transcribe truth."

Memory aid: a roof (宀) sheltering the act of transcription. The simplified 写 keeps only the roof and a hint of what was beneath.