In what way can any social media company "censor" someone?

I don't read Twitter much. I still get plenty of information from Facebook, Podcasts, Hacker News, random blogs and websites, news websites, Discord chats, YouTube, etc.

Even if Twitter decided to kick out someone, all of those other avenues are available to them, some of which aren't under the control of almost anyone.

It feels to me like we're living in the age where it's absolutely easiest to "publish" anything at all you want, and yet people are constantly complaining how hard it is. I just don't get it, it seems factually wrong.