If you own a sandbox, you've most likely encountered this situation:
Your sandbox and a competitor platform are really, really good friends. Your users love both of you, they want to share stuff with both of you, and so the idea to "partner" shows up. And eventually it gets turned into something very much real...
A discord post.
That's about it, really. Practically all partnerships boil down to both platforms sending each other an invite link and saying "We've partnered! They're friends...