This question already has an answer here:
In regards to bypassing damage immunity to attacks not made by magic weapon, what are considered magical attacks besides spells/spell-like ability and magical weapon attacks?
Are summoned monsters considered magical? What about bite/claw from dragons and high CR creatures or legendary creatures?
In the old AD&D, there was a rule that said that attacks from monsters of 4D+4 hit dice or higher are considered magical in regards to hitting creatures like werewolves. I also remember reading years ago that attacks from creatures from other planes and attacks from creatures immune to non-magical attacks are also magical. But I can find no such rule in 5e. Does this mean that werewolves fighting in wolf form cannot hurt each other, and that a typical CR9 fire giant has no chance to defeat a CR2 wererat? How about the bite of the Tarrasque?