Kernel
The heart of an operating system — runs in privileged mode (ring 0), manages CPU scheduling, memory, drivers, file systems, system calls. Everything passes through the kernel.
Two architectures: monolithic (Linux, Windows NT — everything in kernel space, fast but large TCB) and microkernel (QNX, seL4, Mach — minimalist, drivers in user space, more secure). Hybrid: macOS XNU = Mach (microkernel base) + BSD monolithic parts. The Linux kernel is ~30M lines of code, ~85% are drivers. Linus Torvalds still maintains large parts personally.