A fun wittle headcanon since tonight was a lot
Reinhardt is canonically a fan of D.va, but how would he know about her?
Brigitte is presumably around D.va's age. Maybe she watches her streams and started watching with Rein one day and it became a bonding thing
Reinhardt is sent on a mission and finds out he's working with the one and only Hana Song! He gets her autograph and realizes that he doesn't have anything for Brigitte!
After the mission is complete, and everyone is relaxing at base, Reinhardt asks D.va if she would mind doing him a favour and calling someone for him
Hana agrees "anything for a fan!" And surprise surprise she's on a video call with Brigitte! They start their conversation out like anyone would when they meet their favourite celebrity "oh wow i can't believe I'm meeting you!" Slowly, the conversation turns more casual, until they're talking like best friends. To a point where they actually exchange information and promise to talk again soon.
The next time Brigitte sees Reinhardt she thanks him for setting the two up to meet and the time after that they're officially dating!