It's a global standard called oembed http://oembed.com here's an example.
You get sent a web link by a friend:
The Textra app then converts this URL to get the 'preview' as follows:
@ http://www.youtube.com/oembed?url= https://www.youtube.com/watch?v=J6a2kheSpT8&format=json
So, in this example, it's STILL YOUTUBE doing the display of the URL, same as if you just tapped it.
Note: We do not show previews for unknown contacts (not in your address book).