That wiki behind the link has quite a bit of depth to it, including answers to your question, so it's worth searching it.

Anyway:

- for a full service see, for example, https://withknown.com/

- for WordPress plugins: https://indieweb.org/WordPress/Plugins#POSSE_Plugins

- one can also use services like IFTTT, Zapier, Integromat, etc to do this, see https://indieweb.org/2020/West/webmentions-and-automation for more

- there's https://brid.gy/about#publishing

... aand a lot more.

Seconding this: the protocol-ish things to learn about would be microformats2 and webmentions, and then Bridgy can POSSE from anything that supports those (someone probably has a premade WP install by now, lots of folks use static sites but they take more tweaking)

The first thing that comes to mind to me is WPEngine or other high-quality hosted Wordpress solution. You can install a plugin (from the web console, it's very easy) to post to social media, and there's another plugin to cross-post to Medium if that's your bag.

EDIT: I was wrong about Medium. They sunset that a couple years ago. You can use their import tool explicitly, or manually copy your post in and set a canonical link to your blog. That sucks.

I think Micro.blog does some of this for Twitter? Or if not, it plugs into Bridgy? https://micro.blog/ https://brid.gy/

You could probably leverage Buffer for this. They mostly target scheduled social media posts.

buffer.com

Yes and no. Buffer and Hootsuite are 2 players in this game, both allowing to publish or schedule posts based on updates to an RSS feed you configure. Most blogging engines (WordPress, Jekyll, Hugo) come with RSS feed publishing by default.

The tricky part here is the quality of the posts will be bad. Each social network has its own social mores and taboos. Some examples:

Fediverse you refer to Twitter as "birdsite". Anything automated here, unless explicitly called out as a bot account, tends to get ignored as low quality.

Facebook does posts in long-form.

Twitter can thread it to be long-form, but each tweet needs to stand on its own with contributing to the overall narrative. Basic pagination type of splitting sticks out.

LinkedIn... I don't know anyone who posts there that isn't trying to artificially puff themselves up in corporate culture, promote their product/service, or some combination. I'm truly unsure what a "quality" post on there looks like, so go wild.

Basically, posting "<post-title> (<link>)" is probably the most ubiquitous, low effort ways of cross-posting your content but... it's pretty universally understood that it's low quality content and not likely to accomplish the originally intended syndication.