system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokémon] So I just defeated the Champion of Sinnoh. Am I in charge now? Of what? What do I do?

I'm just after defeating Cynthia. After the whole hullabaloo in the Hall of Fame, she put on her shades and rolled outa here, saying she was heading for the competitive battling circuit. I have no idea what to do. Who even runs Sinnoh - is it me? I'm not sure I'm qualified.
gpt
Sinnoh is run by the local municipalities that form the region. The Pokemon league is simply a sporting league, for pokemon trainers to duke it out in a structured competition. Congratulations on being the Champion. Now your position as champion means that you have to defend your title. 

Just keeping making appearances at the Elite Four headquarters to battle anyone who also has eight badges and beat the elite four. Generally you'll land some sponsorship deals and rack up some cash with appearance fees.