僧 is an abbreviation of 僧伽, the transliteration of Sanskrit 'sangha' meaning the Buddhist monastic community. In Chinese, 僧 refers specifically to male monks, while 尼 refers to nuns.
Person radical on the left flags 僧 as a kind of person - specifically a Buddhist monk, a transliteration of Sanskrit saṃgha (community). Same radical anchors it alongside other religious-role chars like 仙 (immortal) and 佛 (Buddha).
曾 gives the sound - céng/zēng → sēng - through a sibilant shift common to this phonetic series. The same 曾 phonetic produces 增 (zēng), 赠 (zèng), and 蹭 (cèng), all sharing the same nasal ending and sibilant onset.
