Brussels Airport (BRU) sits close to the centre and serves most full-service carriers, while Brussels South Charleroi Airport (CRL), well outside the city, serves a number of budget airlines; the airport a booking arrives at can meaningfully change the first transfer, and is worth confirming before the district is chosen.
Brussels is not one city but two identities layered on the same streets — a small, bilingual historic capital of guild halls and comic-strip murals, and the unelected administrative capital of the European Union, its Kirchberg-style towers a short walk from the Grand-Place. A plan that accounts for which version of the city a given evening belongs to tends to move more coherently than one that treats the two as interchangeable.