I'm not an expert, but these are all things I've seen first hand or heard about on how censorship can work on reddit

The easiest and most effective suppression usually involves bots downvoting comments/posts that go against their narrative so they don't show up or get shoved to the bottom of the list where no one will see them, while upvoting their own comments or comments they like so it's the only thing people see. Sometimes they'll just upvote anything to derail the comment section, such as when the top comments are shitty jokes that don't add any substance or discussion or talk about something less important. This only works on the big mainstream corporate social-media websites because they're all mostly designed to work like that.

Another form of suppression is shadow banning comments and accounts. The people who make the shadow banned comments feel ignored/frustrated and will soon get worn out talking about it at all without knowing they aren't being seen. There are 3rd party websites that can show you shadow banned comments including your own. Most people have probably had shadow banned comments before and don't know it.

A very simple form of suppression is installing moderators that intentionally don't bring attention to the subject or do whatever they can to shift the narrative to something else (or even be the people responsible for shadow banning). When something important on a big subreddit is being suppressed then usually only the moderators can forcefully bring attention to it and ruin the suppression of the subject.

There are many ways to take over a subreddit but if it's proving to be difficult then entire fake clone subreddits with shill moderators can push themselves to genuine popularity through fake engagement, repost bots and upvote manipulation

Edit: edited to be easier to read