A lot of people don't understand the difference between local sports teams and national sports teams.
The NBA is a professional basketball league that is made up of LOCAL basketball teams in 30 cities across North America (29 in the US and 1 in Canada). NBA teams represent their respective cities and/or regions, but NOT the country that they play in. Thus, NBA teams are NOT "World Champions". For example, the Lakers do NOT represent the entire United States; they only represent Los Angeles and the greater Los Angeles area.
Basketball teams in FIBA and the Olympics are NATIONAL teams because they represent their respective countries, and the players on those teams are solely citizens of that respective country. FIBA and Olympic teams also play against national teams from other countries. Thus, whoever wins the FIBA World Cup or Olympic basketball tournament can be considered "World Champions". Team USA in both FIBA and the Olympics represents the entire US in basketball.
Another way of looking at it is that even though the Toronto Raptors play in Toronto, most Raptors players are American. That's because the Raptors are NOT Team Canada, and Raptors players do NOT actually have to be from Toronto or Canada in general. Meanwhile, Team France, being a national team, will ONLY be made up of players who are French (meaning they are citizens of France).
The same is true in Euroleague
Real Madrid are the reigning champions and their starters were from America, Bosnia, Hungary, Croatia and Cape Verde. Only 5 players on the roster were actually from Spain
Only countries where this is not the case are the ones like China where their rules limit the amount of foreign players allowed on a team
Everyone should know that players on a club team typically are from all over