FIBA does tournaments for different countries, it isn't really a league.
en.m.wikipedia.org
I think you're getting it confused with the different leagues like the Euroleague and Eurocup which are leagues for some of the top teams throughout Europe. Those leagues have privately owned teams from different counties throughout Europe/Euro-Asia that earn their way to play in the leagues.
en.m.wikipedia.org
It's also a ton of really good dometic leagues like the Liga (Spanish) ACB and adriatic league that have nothing but teams in their area that play against each other. For example Real Madrid and FC Barcelona plays in this league against each other due to both teams being in Spain or Partizan vs Crvena Zvezda both being Serbian teams. The adriatic league has teams where the countries are located close to the adriatic sea so they also have teams like KK Cibona which is located in Croatia.
en.m.wikipedia.org
en.m.wikipedia.org
All these teams play in multiple leagues if they qualify for them and of course not all leagues talent level are equal.
The consensus best overall league is the Euroleague and the best domestic league is Liga (Spanish) ACB, those are the two leagues Luka Doncic played in and won an MVP as a teenager.
