I am doing an assignment for University and I need to install Mac OS Sierra. I have been looking for installing it, but all places I found are not official. So, do you know how can I achieve a .iso for Mac OS Sierra officially?My professor says something about that the kernel is free distributed, but the GUI doesn't. It is for installing in VirtualBox.
Get Mac OS Sierra for VirtualBox
osx;virtualbox
From the macOS Sierra license:... you are granted a limited, non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at any one time.As I mentioned in a comment, various parts of the operating system are open-source and available for download.