MBTI exploded as Chinese internet small-talk currency around 2022 — replacing
星座 (xīngzuò) 'astrology' as the default get-to-know-you label. The 16 type codes (INFP, ENTJ etc.) are used as nouns:
我是 INFP. The single-letter slang I
人 (I-rén) 'introvert' and E
人 (E-rén) 'extrovert' come from the I/E axis.