How does fusion impact the ball a Pokemon is caught in?

[MxRobinson]

I'm replaying SwSh right now and I'm about to catch Calyrex. The one you get during the story will always share its ball with its horse, but if I transferred a horse in a different ball to the game would their fusion use Calyrex's ball or the horse's ball? Sorry if this is common knowledge, it's kinda hard to search for anything related to fusion without running into a bunch of fanmade stuff now lol

 

[Finnematics]

I believe it will always use Calyrex's ball, because the fused Pokemon is still Calyrex. Same should apply for Kyurem and Necrozma