If you are using the "Publicize" feature and it's been set up to link to your social media account, then posts should automatically post to each of your accounts when you publish a new one.
This will only work if the post has not previously been published on your site. (e.g., They won't resubmit to facebook, if you republish a post). This is to prevent blog posts repeatedly posting to social media when you make minor edits and updates.
To publish manually, you can navigate to the post on your live site and click the facebook icon below the post, then follow the instructions.
If your posts are not appearing on your social media account, then try re-authorising your site to post. (For example, for facebook, On the right of the post edit page - under the heading "Publicize", click on settings. In the screen that opens, remove the facebook link, then relink by clicking "Add new" under facebook and reconnect the account.)
Facebook accounts managed by Facebook Business Manager can not utilize the auto-posting feature.
To manually post an item to social media, open the webpage/post to share on the public-facing site. Click the icon link to share at the bottom of the page.