That’s an impressive board! Is that solid - I could not tell if it was a veneer.
It is solid maple with strips glued in the cut out curves.
Ah ok - that’s some precise curves! Freehand?
Well, sort of. I draw a curve by hand that appeals to me, then I make a template that is that curve. I use that as a guide for a router bit that is the thickness I want, in this case 1/2", 3/8" and 1/4". Rout the 1/2" gap first only about 1/4" deep, cut into 2 on the band saw and use a pattern bit to smooth out the cut. Make up the layered inset and glue it up. Clean it up and repeat for the other s. Nothing to it!