她 = 女 (woman) + 也. Notice it's just 他 with the person radical swapped for the woman radical 女. Same sound (tā), female meaning.
Here's the surprise: 她 is barely 100 years old. Until the 1920s, Chinese wrote 他 for everyone — he, she, it. The poet and linguist Liu Bannong (刘半农) coined 她 around 1920 to translate Western texts that distinguished 'she'. It caused fierce debate at the time; now it's everywhere. You're learning one of the youngest characters in the language.
Woman (女) + 也 — the female version of 他: SHE.
她 was invented around 1920 by Liu Bannong — making it one of the newest Chinese characters. For 3,000 years before that, 他 meant he AND she.