Is there a manual for Jai, or some info on how it compares with other languages? I find it hard to find resources on it - there's no Wikipedia page, and my web searches list some videos and random github projects?

It's closed to the public at the moment, so I don't think you will be able to find much.

The closest thing would be the book that Ivo Balbaert is writing and that's available here: https://github.com/Ivo-Balbaert/The_Way_to_Jai

this is also good: https://github.com/Jai-Community/Jai-Community-Library/wiki

https://github.com/Jai-Community/Jai-Community-Library/wiki/...

https://github.com/Jai-Community/Encyclopedia-of-Jai-Example...

https://github.com/Ivo-Balbaert/The_Way_to_Jai/