system
A chat on subreddit /r/wallstreetbets
human
They're competitors if they're in the same industry and offer a similar service. Walmart and Amazon are competitors, and people shop at both.   
  
 Disney+ is a competitor with Netflix, just like Hulu/Amazon Prime/and later Apple's/HBO GO service.  People can have none, one, a few, or all of them. Parents with young children might decide to go with Disney+ and drop Netflix, etc.

gpt
Good points, I was thinking about it from the 'economics' perspective, like of goods that are used together or affect the demand of each other (classical example being beef and hamburger buns demand correlates so therefore they are complimentary) but I see what you're saying and I'd have to agree (although I don't think people will substitute their NFLX subscriptions with DIS)