I don’t think Tanenbaum’s distinction between micro-kernel and monolith is useful or important. He has monolith as a single binary running as a single process,

I don’t think Tanenbaum’s distinction between micro-kernel and monolith is useful or important. He has monolith as a single binary running as a single process, while micro-kernel is multiple binaries/processes. But either way these both boil down to bytes loaded in memory, being executed by the cpu. The significant thing about a microkernel is that the operating system is organized into functional parts that are separate and only talk to each other via specific, well defined channels/interfaces. Microkernel uses processes and messages for this, but that’s hardly the only way to do it, and can certainly be done in a bunch of units that happen to be packaged into the same file and process. C header files to define interface, C ABI to structure the channels, .c files for the separate pieces. Of course you could do that wrong, but you could also do it right (and, of course, the same is true of processes and messages). A process, btw, is an abstraction implemented by the os, so microkernel or not, the os is setting the rules it plays by (subject to what the CPU provides/allows).