Kernel

The kernel is the core part of an (OS), responsible for essential resource allocation, low-level hardware interfaces, security, etc.

Kernel types
A monolithic kernel runs all OS services in a single thread. Developers may find this easier to implement, as was done by Apple Computer with the original Mac OS for 68k processors (up to System 7.1.1). However, this makes the system vulnerable as a bug in a device driver could cause it to crash entirely.

A microkernel is an approach to operating system design emphasizing modules that implement basic features as individual processes that run independently of the core system kernel and can be more flexibly configured. Apple attempted to implement this with Copland's NuKernel, but failed.

A nanokernel is a very small kernel that offers hardware abstraction and few or no other services. The PowerPC versions of classic Mac OS 7.1.2 and later were implemented in this manner with a tightly integrated Mac 68k emulator running a largely unported operating system on top of it. This Mac OS nanokernel was not rewritten to be PowerPC native until Mac OS 8.6.

A hybrid kernel combines the features and benefits of a microkernel with that of a monolithic kernel. The XNU kernel of Mac OS X (now macOS) and its derivatives, such as iOS, adopts this method.