They made a calf (emoschopoiēsan). First aorist active indicative of moschopoieō, here only in the N.T. and unknown elsewhere. The lxx (Exodus 32:3) has epoiēse moschon from which phrase the word is evidently made. Aaron made the calf, but so did the people (Exo 32:35).
The idol (tōi eidōlōi). Stephen calls it by the right name. The people said it was their way of worshipping Jehovah! So the Egyptians worshipped the bull Apis at Memphis as the symbol of Osiris (the sun). They had another sacred bull Mnevis at Leontopolis. Eidōlon (from eidos, form or figure) is the image or likeness of anything. The heathen worship the god through the image or idol.
Rejoiced (euphrainonto). Imperfect, middle, kept on rejoicing (Exo 32:6, Exo 32:18) or making merry.