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Drivers
At the last count, device drivers accounted for over 80% of the code in the Linux source tree. They're a very important part of any operating system. Without drivers, an operating system can't do very much.
Drivers in Symbian are usually split into two parts. The top half provides the interface to userspace, and the bottom half provides the interface to the hardware. The top half can be shared among all of the drivers in a particular category (such as all display drivers), while the bottom half is device-specific. Often, there's a third part, which isn't really part of the driver. The driver and kernel often won't handle multiplexing the device at all; they'll grant exclusive access to it to a single userspace server (in the TCE), which then accepts requests from other programs with the correct privileges.
Most drivers are provided as shared libraries, and loading them uses the same capabilities mechanism as any other shared library. The kernel process will load them only if they have the TCB capability, along with any other capabilities that the kernel has. This option isn't possible for all drivers, however. Some drivers are needed by the kernel before it finishes booting. In particular, the application-specific standard product (ASSP) and variant drivers define the core system. They're external to the core kernel, which just defines the CPU architecture, not the whole system on chip. Like the kernel itself, these are typically execute-in-place binaries that are loaded before the kernel finishes initializing. This approach allows manufacturers to create a platform-specific kernel without having to recompile the core kernel. (This feature was probably a lot more important before Symbian was open-sourced.)