FYI
After Sept. 18, you’ll no longer be able to send or receive direct messages on YouTube as we focus our efforts on public convo tools (comments, posts, etc).
You can still share what you’re watching w/ others via the Share icon below videos.
More→ https://yt.be/help/9Hns
You're thinking about the channel one. The other one was for friend messaging. which is at the top right of the page next to the Notification bell icon. (Desktop)