Barcelona can be said to be the top European giants, winning the Champions League and La Liga titles many times, which also makes Barcelona famous in the world, and Barcelona is very attractive to players. Many players will choose to play in Barcelona in order to make themselves more famous and develop. This also keeps Barcelona strong in a sense.
Juventus have not won the Champions League these years, but they have also successfully reached the final of the Champions League. This is a success in itself, and Juventus has won eight consecutive titles in Serie A, which also proves that Juventus is a very good team and its defensive ability is also very good. Coupled with the joining of Cristiano Ronaldo, Juventus' combat effectiveness is getting stronger and stronger.
Of course, there is not much basis for this assumption, but both sides will meet some giants from other countries in the Champions League. In the face of big teams from other countries, these teams often have excellent performances. So from this perspective, the two clubs will still have a very good level of playing other leagues.