Hashtags have a strong effect in social media and youtube, but they acquired more power now in YouTube for its effect on promoting youtube channels and videos.
- Where can I write Hashtags in my video ?
Hashtags can be written in the description box of a video, they appear above your title when someone click on your video, which will make your channel ranked up in search engines.
You may see this video for more info:
and this video for more details about : How hashtags work on your channel.
All the best.