After Apple acquired NeXT, the Mach component was upgraded to 3.0, the BSD components were upgraded with code from the FreeBSD project and the Driver Kit was replaced with a C++ API for writing drivers called I/O Kit.
Kernel design
XNU is a hybrid kernel, containing features of both monolithic kernels and microkernels, attempting to make the best use of both technologies, such as the message passing capability of microkernels enabling greater modularity and larger portions of the OS to benefit from protected memory, as well as retaining the speed of monolithic kernels for certain critical tasks.
iPhone giant compartmentalizes OS for the sake of security. Apple has been working to harden the XNU kernel that powers its various operating systems, including iOS and macOS, with a feature called "exclaves." ... .
