Brussels = location of EU headquarters, and in common lingo Brussels thus means "People running the EU and deciding things on everyone's behalf."

"Brussels" is often used to mean the entire blob of EU and related institutions.

It is called a metonymy where you substitute a name that is associated to some other thing instead of mentionning that thing.

Some examples: Wall Street = NY Stock Exchange the White House = US president and his cabinet the Pentagon = US Dept of Defense Downing Street = UK prime minister

https://en.wikipedia.org/wiki/Metonymy Scotland Yard = Greater London Metropolitan Police Tehran = Government/officials of the islamic republic of Iran