Tap
next to the video to show the video’s comments, description, date added, and other information.

On the More Info screen, tap “Rate, Comment, or Flag,” then choose “Rate or Comment.” You must be signed in to a YouTube account to use this feature.
On the More Info screen, tap More Videos.
On the More Info screen, tap More Videos, then tap “Subscribe to <account>” at the bottom of the video list. You must be signed in to a YouTube account to use this feature.