In North America and Europe it's theoretically open but OEM's dependence on Google's services (which for many people are part and parcel of the Android experience) and the control Google exert through that means that there are real limitations on how open it is in reality. This week showed just how much freedom Samsung, the largest smartphone maker in the world, has to do what it wants with Android - almost none.
You can argue that that's not really Android, it's the services, but until there are solid FOSS alternatives to those services you can't use Android to put together a compelling smartphone without Google.
That may change over time but for now for a majority of people it's a relatively closed platform.
In China and much of Asia where Google services are either unavailable or less compelling it is far closer to open. OEMs use the OS, bundle what they want and ship it without Google knowing or controlling anything and that's maybe a third of Android shipments worldwide so not insignificant.