According to this video, the [u] letter as in Hund [hʊnt] is the short version of [o:] instead of [u:], but as I know, there is another letter [o] as in Millionär [mɪli̯oˈnɛ:ɐ̯] being the short version of the same letter [o:].
So my question is, what's the difference between [o] and [u], if they're the same thing why do they use two different IPA letters to represent one single vowel?
EDIT: As the video mentioned above doesn't use the standard IPA script, I have to clarify my question here. What confuses me is these two letters in IPA script [o] and [ʊ]. Some would say that the short o sound is denoted by [ɔ] as in Nonne [ˈnɔnə] , but there is another one letter [o] as in this video.