In monster statblocks that include resistance to bludgeoning, piercing, and slashing it is stated that you can overcome that resistance with magical weapons or sometimes silvered weapons.
In the Barbarian rage description, nothing like that is mentioned. But I always thought that magic weapons always ignore that resistance and I am wondering why some monsters including something like Demogorgon:
Magic Weapons. Demogorgon's weapon attacks are magical.
Maybe that's just flavor so Demogorgon is just better against other monsters or those attacks also ignore the resistance that the barbarian has while raging.
Do raging barbarians have resistance against attacks made with a magic weapon or a weapon that counts as magical to overcome resistance?