@MilanMarecek Yes, the interface does need some work, but I thought the same thing when I followed the instructions given earlier. But let me provide some more details.
1. You have to be owner or admin
2. You have open the video (meaning, it can't be done bulk sadly)
3. Once you open the video, click three-dots, then Add to group/Channel'
4. Under Share With, Click People, and Select Channel.
5. Then type the name of the Channel you create and press search.
6. Click Save.
The same can be done under "Update Video Details".
1. Click Update Video Details
2. Under Share With, Click People, and Select Channel.
3. Then type the name of the Channel you create and press search (OR "Create a Channel" also comes up if you haven't created it before).
4. Click Apply (Apply is up top this time when you're using update video details)
Lastly, it can be Done from the "My Content" heading by click on the correct icon (See last screenshot below).
I added some screenshots below:
