I don’t remember ever joining a team, nor do I remember wanting to play in Tournaments. How can ..

If you tap the team tournament shield and then tap the “my team” tab at the top of your screen, there’s a green info button on the top right - tap that and you will see the “remove” option to leave the team. The other tournament for individual players is automatic. If you play during tournament times your progress will automatically be tallied among the other players. It does not affect you though - just play when you want and if you don’t care about your progress in the tournament, you don’t ever have to check it or feel the need to actively compete for top placement.