@deborahh No, a written post is not automatically a creation/art. (If it is completely public, Media can eg embed it by code without asking. May not if it is not public.) But Mastodon says: We don't grab it for AI. Meta does!
Photos have a copyright and Meta especially grabs art from artists. It's a big debate at the moment how artists can protect their art or sue AI companies.
Or if you post a photo of your baby: Meta uses it to train AI. If you join FB or Insta, it's in the GTAC. Not here.