Is it just / ?
I kid. But really, besides “its all a file”, if you take away the gui, is the only difference the syntax ? How libraries interact? How disks are mounted ?
If we stripped all ms’s junk out and made windows open source, would we still prefer linux?
When you get to a very basic level, is one of them more efficiently coded?
At a super basic level, MacOS, Linux, and Windows all use different kernels.
From a very basic viewpoint. The kernel is basically the piece of software that gives each piece of hardware their instructions.
At the absolute most basic level, assuming equal hardware, all the kernels are working with same set of instructions(store this value in this register, move to this location in the stack, read this value, etc) but they issue those instructions in different orders to achieve very different results.
The NT kernel that windows uses does basically everything in a different way than the Linux kernel. To the point that their only real similarities are that they speak the same language to the bare metal hardware.