There are three related questions that link to each other and have similar answers:
- How does Ancestral Protectors' resistance work against multiple damage types?
- Does a rogue's Uncanny Dodge halve each type of damage individually or the total?
- Multiple Damage Resistance
They each (effectively) ask how damage resistance should be applied when something deals multiple damage types. Should these questions be marked as duplicates?
Some things to consider in choosing a potential duplicate: the third question in the oldest, its answer has the most upvotes; the second has the most views; the first and second were actually asked on the same day.