What do you mean by open and closed? ChromeOS is based on ChromiumOS, which is open source. I guess macOS is based on Darwin technically, but the ratio of open source to proprietary is much higher for ChromeOS than macOS, no?
I think they mean closed as in it is more difficult to install whatever you want on a ChromeOS machine as opposed to a MacOS machine.
I think they mean closed as in it is more difficult to install whatever you want on a ChromeOS machine as opposed to a MacOS machine.