What are best practices for search engine optimization of YouTube video content? By that I mean filling in descriptions, titles, tags and so on. I can't find good information about this on the internet, about which keywords to use, optimal/maximum lenght of texts and other (?) issues
The things you mentioned are a good start.
Something that is fairly key/essential is to make sure you also provide a transcription (weren't Youtube trialling an automated way of doing this at some point?) since obviously search engines cannot determine words from audio to be taken into account when ranking content (for now, at least).
Keyword - YouTube will start suggesting keywords as you fill-in content.
I would recommend you to read Vlogger Academy's Youtube SEO guide. It will help you to understand the basics of video seo on YT. After you finished the base guide, also read the detailed articles about video tagging, description and video title writing.
In a nutshell, please check the following ranking factors that I copied from the linked material:
- Video title
- Video description
- Video tags
- Video views
- Video comments
- Video likes
- Subscriber number