how cells form into specific shapes for arms, hands, organs, etc.
Not really qualified to answer, but Michael Levin have done some very interesting work on bioelectricity and growth of bodyparts. He is beginning to control the cell/formation of bodyparts, and other fascinating stuff. There’s some fine interviews on yt.
(Just for fun!)
Oh, just noticed that. Anyway, you seem interested in the subject, so you should check him out anyway ;)