How do magical weapons bypass non-magical attacks?
For example, a lot of monsters with resistance or immunity to melee damage types will say „from nonmagical attacks“. In this case, magical weapons do indeed bypass this resistance/immunity. Swarms (i.e. Swarm of Insects), on the other hand, do not; they just say they have resistance to „bludgeoning, piecing and slashing“.
Is there a resistance to magic weapons or attacks?
No, unless the Statblock says otherwise In 5e, there is no such thing as Resistance to Magic Weapons or Attacks (not that I’ve seen in any statblock, anyway).
Can You wield a quarterstaff as a magical weapon?
Several magic staves have a sentence beginning: „This staff can be wielded as a magic quarterstaff…“. Magic quarterstaves are magical weapons. Magic staves that do not have this sentence do not qualify as magical weapons, even if you use one as a quarterstaff, any more than punching someone with a magic glove counts as a magical unarmed strike.
Do magical weapons automatically overcome damage reduction?
The general question here is whether magical weapons automatically overcome damage reduction. My read is that magical weapons can overcome DR, but it’s not guaranteed. Is that correct? Correct on both counts. See the PFSRD.
Can a +3 weapon overcome Dr/adamantine without being adamantine?
+4 and greater weapons overcome DR as if made from adamantine +5 weapons overcome DR as if aligned Thus, a +3 weapon over comes DR that is vulnerable to magic, cold iron, or silver, but not adamantine (unless it was actually made from adamantine). It would need +4 to overcome DR/adamantine without being adamantine.
What level of magic can penetrate Dr 10/magic?
Any degree of magic will penetrate DR 10/magic. Also, in 3.5e, you need an actual Silver weapon to penetrate DR 10/silver. Magic iron is just as reduced as the regular stuff. 10/magic means that magic weapons are required (of any plus).