Sneak attack says:
Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
Would the extra damage from sneak attack bypass the resistance of a creature that had damage resistance/immunity to damage from weapons?
Damage Immunities: Bludgeoning, piercing, and slashing from nonmagical weapons
One aspect I am specifically curious about is that sneak attack does not say that it is weapon damage (nor does it say it isn't); all it says is that it is "extra damage". Is sneak attack considered to be damage from a weapon or is it something separate?
For example, consider the case of a rogue with a nonmagical dagger hitting a creature with the above damage immunity. Obviously the dagger does not do any damage, but what about the sneak attack damage? What about when the creature only has resistance?
Resistance/Immunity would apply (because the rule used in the question is defunct)
It's debatable whether or not sneak attack's damage is from the weapon: but one thing I can be sure of is that sneak attack damage is from the attack. This can be seen in the PHB (p. 196, bold added), where it states:
That's very important, because the rule you're quoting about "nonmagical weapons" is now defunct. See the Monster Manual Errata which states:
The same errata goes further, explaining what magical attacks actually are (and implies what they aren't):
And rules designers have explicitly stated that sneak attack is intended to do the same type of damage as the weapon used in the attack. Jeremy Crawford has stated:
So although it may be debatable whether or not sneak attack damage comes from the rogue's weapon, it definitely is part of an attack and will do "bludgeoning, piercing, or slashing" damage if that's the type of damage done by your weapon. And so, if the weapon used in this attack is a nonmagical dagger, sneak attack is part of a nonmagical attack and would deal piercing damage, and thus the resistance/immunity to "piercing... from a nonmagical attack" would apply.