What is considered a drop?
What does the drop rate buff affect exactly?
Are the ores we mine from ore veins considered drops?

Items dropped from killing monsters.

The drop rate only affects drops from monsters. If something had 20% chance to drop normally and there was a 50% drop increase buff active, that drop would increase to 30% chance.

The items you obtain from mining ores or gathering from plants are not affected by this buff. They have a fixed chance of their own, completely separate from the usual monster drops.