Why are huge sections magnetic?

If it is magnetic in lines going all the way across the ceiling: you have drywall hanging on resilient channel. Great for soundproofing! Unlikely you actually have this. These can be used as your stud.

If it is a huge magnetic section about 16" or 24" across and goes along the entire ceiling (measure where the magnet holds and releases): this is a sheet metal joist cavity air return duct for your floor or the floor above. These are done by slapping some sheet metal between joists and calling it a return duct. If you suspect this is the case, measure the distance between where the magnet lets go and the edges are your joists.