The Mach micro-kernel was originally a BSD kernel fork (iirc 4.2BSD, when dinosaurs roamed the earth). NeXT/Apple added various things like DriverKit and later IOKit to Mach. Apple also took the network stack from FreeBSD, which is traditionally rather modular (for high performance kernel code, anyway).

Together they make up the current XNU hybrid kernel. You can look at most of the sources at opensource.apple.com

There's certainly lots of FreeBSD in OSX, but it's not like OSX is just FreeBSD with a sugar coated Apple UI.