The issue is not that x86 is mentioned. It's just that there is a lot more to an operating system than just getting code running on a CPU.
RPC, the input stack, the graphics stack, the network stack, audio, profiling, telemetry, scheduling, UI toolkits, security, service management, application management, etc.
If you want to learn these things you will need to read the documentation and source code for other OSs.
RPC, the input stack, the graphics stack, the network stack, audio, profiling, telemetry, scheduling, UI toolkits, security, service management, application management, etc.
If you want to learn these things you will need to read the documentation and source code for other OSs.