We’ve seen that je suis can contract in informal language to what sounds like chu or chui. But where does that ch sound come from in chu and chui, considering there isn’t any ch sound in je suis to begin with?
To get the pronunciation chui, je suis contracts to j’suis. Je loses its vowel sound, and the resulting j’s makes the French ch sound. The same thing happens with chu, which is je suis contracted to j’su’s.
So the ch sound comes from j’s.
This is why je sais contracted informally to j’sais sounds like ché. It’s also why je sois contracted to j’sois sounds like choi. Je savais contracted to j’savais sounds like chavais. Anywhere you have the informally contracted j’s, you have the ch sound.
Cynthia Dulude uses the pronunciations j’su’s (chu) and j’sois (choi) in one of her videos in the Listen section.
i think what’s going on here is regressive assimilation where due to the dropping of the vowel ‘e’, we are left with a voiced consonant ‘j’ next to a voiceless ‘s’ , so the regressive assimiation causes the voiced ‘j’ to become voiceless ‘ch’