Several parts combine into one character.

内 combines 冂 (a cover, a boundary) with 入 (to enter) going inside it — to enter within a boundary, hence "inside, within." In the modern form the 入 looks like 人, but it was originally 入.

Mandarin: nèi, 4th tone. 内 (nèi, inside), 内容 (nèiróng, content), 国内 (guónèi, domestic), 内向 (nèixiàng, introverted). It pairs with 外 (wài, outside) — 内外 (nèiwài, inside and out).

Japanese: on-readings ナイ (nai) and ダイ (dai), kun うち (uchi). 案内 (annai, guidance), 内緒 (naisho, a secret), 身内 (miuchi, one's relatives / insiders). The Japanese うち also carries a sense of belonging — "my house, my side" — forming the deep cultural distinction between 内 (uchi, in-group) and 外 (soto, out-group).

Korean hanja-sound "nae": 內部 (naebu, interior), 室內 (sillae, indoors), 案內 (annae, guidance). It is the most basic spatial word, paired with 外 (oe, outside).

Memory aid: 入 (entering) going under a cover (冂) — the inner side of a boundary.