I think Udemy Community Managers cherry-pick the most helpful and resourceful people among the community to give them the community champion status, the purple star and badge.
There are no hard and fast rule that I can find to get the community champion. But here are my thoughts.
1. Get all those other badges by getting likes, starting topics and replying to peoples query. There is clarity on how to get them, so get them first.
2. I also find almost all the community champions are also published authors on Udemy. So get your content out on Udemy.
3. These guys are also super consistently active on the community, most of them post every single day. So be active.
4. While doing all of those above, try to be in spotlight section "Top Liked Authors". So that the community managers will realize, oh there is this person we should consider while preparing the next set of community champions list.
Here is a post on Community Champions which might be helpful to know more about each Champions.