Every chorus battle has its own set of requirements, and there is no central entity that governs battles. Any group or individual can organize one, therefore there is no single place, requirements, etc, as everything is decided by the organizers.
For example, you can organize a chorus battle. To do that you will want to think of a theme, requirements/rules, prizes. Then you will want to create a teaser video, with more information, a structuree google doc with more detailed information, a way for people to sign up (e.g. a google forms), and advertise maybe on twitter or utaite forums.
Youtaite who are interested in your chorus battle will then join and the battle commences.
Your best bet to know about upcoming chorus battles would be to join some kind of chat or forum where other youtaite tend to announce things like that. Maybe also follow some youtaite, that may retweet announcements to chorus battles.
For all requirements on a certain chorus battle, check out their teaser/introductory video and the video description. Most likely you will find the google doc with more information in the description of that video.