It's a live streaming site that got it's original stride through game streaming. It's basically watching people play games. Like watching sports. Part of what made Twitch attractive for users on the platform was subscription and donation services. You could pay a certain amount of money to be subscribed to a streamer so you'd never have to miss their content(So like cable, but way worse)

Eventually some girls of certain mindset recognized that they could make a decent chunk of cash by acting like gamer girls while sitting there, knowing lonely men would start dishing out money in hopes of interaction or something more. Eventually they dropped the fake gaming all together and allowed a just chatting, but still had some regulation on what the girls could wear or do on stream. (since the site isn't adult restricted it has to maintain some standard)

Well, some girls pushing the envelope figured out that the places where bikini attire is generally acceptable can be streamed, which amplified the cash flow.

People still have twitch streams for gaming, reacting to various entertainment media, slice of life livestreaming, etc. But twitch has a THOT problem and the simps are eager to open their wallets for a "chance."