Matchup players & teams are found in the Playgrounds they are registered with. A player found in Playground ‘A’ could also be listed in Playground ‘B’. Hence do select the venue for the Matchup before picking who you wish to play against.
The playgrounds listed in Select Matchup screen are the Playgrounds you are registered to.
If you wish to have a friendly Matchup with a friend who do not have a common Playground, simply register that Playground via Search or via your friend’s Profile screen (Vs icon).