John Gill Commentary Exodus 32:16

"And the tables were the work of God, and the writing was the writing of God, graven upon the tables." — Exodus 32:16 (ASV)

And the tables were the work of God
And not of angels or men; the stones were made and formed by God into the shape they were:

and the writing was the writing of God, graven upon the tables ;
the letters in which the law was written were of his framing, devising, and engraving; and this was to show that this law was his own, and contained his mind and will; and to give the greater dignity and authority to it, and to deter men from breaking it.