"Each driver should be a process or at least a thread of a system process, right?"
Sorry, no, that isn't true. The "System" process does run some code on behalf of some drivers, but not for most drivers. Aside from a few specifically created "system threads" or "kernel threads", drivers do not run in processes or threads of their own.
Process Explorer will show them to you. Select the "System" process, then View | Show Lower Pane, then View | Lower Pane View | DLLs. In general every module called anything.sys is a kernel mode driver. If you enable the "Base" (address) column you'll see that they're all up in kernel address space.