Welcome here, Akai! 
I’m not sure what you mean by “seems to be closed” and “seems to be an actual forum” 
Customization > Theme and Customization > Theme component are both categories that have the same security settings.
You can reply to existing topics, but to post your own theme or theme component, you must belong to the Theme authors - Discourse Meta group
Their page appearance differs as you may have noticed, but they are both topic lists.
Themes will change the whole appearance of your forum, whereas theme components usually add new features.
It may help depending on what you want to do. Discourse has its own JS API. Since all themes and theme components on meta are open source, you can look at their code to see how they work.
Feel free to ask questions in Support or Development if you need help after reading the proper documentation