? Generally speaking, in Europe, are there many fans who support their own league clubs when watching the Champions League?

To tell the truth, not much. Except for the national team, most fans in club games are extreme. You said that Spanish fans support Barcelona and Real Madrid together, which is more of a case.

First of all, there is no patriotism, just referring to Catalonia's independence.

Secondly, Barcelona and Real Madrid are opposites. Even in the Champions League, the positions of the two sides are opposite, so fans are naturally opposed. Besides, the strongest controversy is inevitable. More than half of Barcelona powder is Messi powder, and more than half of Real Madrid powder is C Ronaldo powder, which is naturally a competitive and independent relationship.