Several parts combine into one character.

當 (traditional) is a phono-semantic character: 田 (field) for meaning and 尙 (still) for sound. Its original sense was "two fields matching in value, corresponding" — from which grew "appropriate, to match, to correspond, to take charge." The shinjitai and simplified cut the upper 尙 to ⺌ over the field, giving 当.

So the form splits: traditional (ko/en/es) is 當, shinjitai (ja) and simplified (zh) are 当.

Korean hanja-sound "dang": 當然 (dangyeon, of course), 該當 (haedang, applicable), 擔當 (damdang, to be in charge), 適當 (jeokdang, appropriate), 當選 (dangseon, to be elected). Korea uses the traditional 當.

Mandarin: dāng, 1st tone (to be / take charge) / dàng, 4th tone (suitable, to pawn). 当然 (dāngrán, of course), 当时 (dāngshí, at that time), 应当 (yīngdāng, ought to), 当作 (dàngzuò, to regard as), 上当 (shàngdàng, to be tricked). A one-character, two-tone case — 适当 (shìdàng, suitable) is 4th tone.

Japanese: on-reading トウ (tō), kun あ.たる (ataru) and あ.てる (ateru). 当たる (ataru, to hit / win), 本当 (hontō, really), 当然 (tōzen, naturally), 担当 (tantō, in charge), 弁当 (bentō, boxed lunch). You meet it daily in 本当 (really) and 弁当 (bentō).

Memory aid: fields (田) matched in value — corresponding, hence appropriate, and hitting the mark.