I guess they're as custom as you want them to be. Higher production bowyers like Black Widow and Bob Lee have standard models you can buy, where all you specify is the length, tiller, weight, grip, and RH/LH. But either of them will make modifications to their standard designs to suit your tastes in type of wood,, etc. if you want. I don't know that either will make you a totally custom bow from the ground up, as they have standard forms, templates, clamps, etc., to produce the various models they offer, but I think the same would be true of most smaller volume bowyers too, other than someone who starts with a piece of wood and makes a totally unique self bow from it.