The high end saddles aren’t really custom, in that they are built from scratch for your horse. Each brand has one tree, and then a certain amount of options that you can pick when you order the saddle. typically, more options for the rider than for the horse.
Ummm, no. True of some brands, sure, but lets not generalize to everyone! Many saddle makers, including high-end saddle makers, have multiple trees to fit a variety of horse shapes. This is an important thing to discuss with your saddle fitter.