you would upload the video on dtube. it would appear both here and on dtube. the votes are cross platform so the same user can only vote once. the earnings displayed on both platforms are the same. they are both using the steem backend. just different websites/interfaces.

so your video will also be posted on steemit so it can get votes from people who dont know about or dont use dtube yet. its all tied in together though. you login to dtube with your username and private posting key. its all built around the same system. just different designs/websites serving different purposes.

you could think of steemit as sort of the mothership of all these apps. any post you make on these other apps will be displayed on steemit as well. but not all posts made on steemit will be available on the other apps.