The term operating system nowadays more strictly means a system of programs designed to allow the sharing of hardware between several programs running on it, including all inter-process control and programming conveniences like file system, device drivers and system libraries. It is an extremely long and involved process to build such an environment, go look up how many lines of code modern OSes have and how complex their architecture is.
Less strictly, it means a slightly rebranded version of an OS already known, such as Windows XP (which is some stuff + Windows 2000, which is some stuff + Windows NT, which is a new kernel (core OS program) + some stuff + some Windows 98 stuff presumably, which is some stuff + Windows 95, which is a rewrite of Windows 3.1, which is some graphical stuff + MS-DOS, which is a rebranded "Quick and Dirty Operating System" (QDOS), which is a rip-off of CP/M, which is a parody of Unix for cheap crappy Intel processors at the time) or BolgenOS (which is Ubuntu with different default wallpapers and some program renames, which is Debian with some fancy usability stuff, which is some package management stuff + various FOSS projects + GNU/Linux OS, which is the GNU project rewrite of Unix libraries + the Linux kernel, an allegedly crappy rewrite of Unix kernel). As you might realise, it is rather easy to slap a new face onto an old OS even without owning a huge company and present it with a new name, especially when automated tools
https://en.wikipedia.org/wiki/List_of_remastering_software exist to these ends.