Tags are quite useful.
Youtube uses a search algorithm that searches for three things, title, meta-tags (description), and tags. Eventually the algorithm also calculates viewer retention and engagement on the video as well but at first that's what it does.
Your official tags should be pretty specific and should be long. If your video is about let's say "Space Engineering and Exploration of Mars" then your tags should not be individual words like space, engineering, exploration, mars, It will only force youtube to but less weight on the important tags and more weight on the tags that have nothing to do with your video.
Creating fewer tags that have more to do with your video such as one whole tag like "Space Engineering on Mars" or "Space Engineering and Exploration of Mars" are a lot more useful and more weight is placed on them.
Meta-Tagging is what is in your description. A healthy longer description is key. You want to repeat the most important keywords in your description as often as possible (it still has to make sense, be logical, and easy to read for readers). It helps youtube look for relevant things so when you have "Space Engineering" in your description like 5 times it makes youtube thing, "Oh wow this video references it like 5 times, it must be more important than these other videos without a good description".
The next important part, is your title. Make sure the title of your video is clear and concise, it also doesn't hurt to possible work in a way to repeat a couple keywords in your video twice.
Another pretty important thing that many people overlook are transcripts or captions. Uploading a full caption set or transcript set or creating accurate transcripts on all your videos have a lot of weight with youtube as well. THIS IS ACTUALLY VERY IMPORTANT
While I may be no expert in SEO I do understand those principles.
As evidence that I know, I deleted my old channel and made a new channel three days ago. I have 57 subscribers and all three of the videos I uploaded are ranked on the first page of youtube when you search for them. The videos I had on my old channel all ranked in the top 4, every single one of them.
Hope I could help, take care everyone and let me know if you have more questions =)