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Is that really what Discord, Whatsapp & co are using to display the embed widgets they have or is it just <meta> tags like I would expect...?

There are several methods they may use:

- OpenGraph (by Facebook, probably used by Whatsapp) – https://ogp.me/

- Schema.org markup (the main point of this blog) – https://schema.org/

- oEmbed (used to embed media in another page, e.g. YouTube videos on a WordPress blog) – https://oembed.com/