Thumb not open

Eh don't worry too much lah! My friend's baby was the same, her PD told her thumb tucked into fist can be normal up to around 5-6 months, and the full palmar grasp (where thumb properly joins in to grab things) only really kicks in closer to 6-7 months. Someone in my mum WhatsApp group mentioned her girl's thumbs only started opening properly after 6 months and PD said still within normal range. That said, since every baby different, best still check with your own PD or a paediatric developmental specialist just to be sure, especially if got other things you're noticing too!