Yeah, the Champion level dancers tend to be familiar with each other but here's a dance where the contestants had never met each other before:
To be fair, I imagine Trendlyon watched videos of Robert and was familiar (in theory) with his signature moves. Robert has danced at the Championship level for over 25 years and every up-and-coming follower knows who he is.
As for how people learn to do that ... think of all the effort you (PNP) have put into the precision of your dance, polishing each micro-movement. Then think of directing that level of effort towards a different goal - observing, interpreting, and responding to another person and to the music.
Ballroom dance training does not focus on the skills needed to excel at West Coast Swing Jack and Jill competition. They're both partner dance and they require some of the same core principles (control, balance, flexibility, etc., etc.). But I recall ballroom lessons being focused on getting all the body parts in the exact correct position for every part of every pattern. In contrast, our WCS lessons often focus on spontaneity and musicality.
The physical communication in WCS tends to occur in only one or two points of contact. (One of the challenges if there is more than one point of contact is determining which is primary and which is supplemental.) As long as information is clearly communicated using the necessary body parts (meaning those particular body parts are in the necessary position), the non-involved body parts can be doing anything that doesn't interfere with the connection. As a follower, I learn to make my body parts (hand, arm, back, ribs, hip) available and responsive to the leader's signals - in the moment. The leader learns which body parts are available and how to use them. The most interesting moves are the ones that are created in response to the current partner and the music, whether or not they have ever been done before.
It's not just a matter of whether the dance is choreographed or not. The WCS circuit has a Showcase division for choreographed routines, and many of the top dancers participate. But the primary focus of WCS judging is response to the music using good technique, and the primary focus of Ballroom judging is perfection of technique in time to the music. If a WCS performance could be danced exactly the same to a different piece of music, it's not Championship level; it's probably not even Intermediate level.
Once you pick you jaw off the floor, review the video closely. Notice Robert's visual leads and how Trendlyon picks them up. Notice when the crowd roars in response to simple moves done perfectly in sync. Notice that both of them are obviously familiar with the music and know exactly when to use certain moves to capture the moment. You don't need to know your partner, you just need to know how to communicate to the music.