Bots and shills make up the most of it. There are a bunch that are paid shills that go around posting talking points. A lot came out about that during the Trump/Clinton election and democrats using CTR and Shareblue. However, there are other interest groups that also hire paid shills to represent their narratives and interests. You'll occasionally even see jobs for "Marketing" that require you to login to social media or user content sites and post things.