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?
The entire difference is the kernel. There’s a reason why Apple moved to BSD forever ago for their OS. Somebody did it better than they would have been able to from scratch. Everything above that level on up to the DE matters less as far as stability and performance goes for desktop users.