Also, resistance doesn’t reduce damage unless the target has resistance to each type of damage from the attack, and then only the weakest of the resistances applies. For example, a character who has resist 10 lightning and resist 5 thunder who takes 15 lightning and thunder damage takes 10 damage because the resistance value to the combined damage types is limited by the lesser of the two resistances.
A creature does damage of a type that is radiant and fire, with ongoing radiant and fire damage. Would a character with only fire resistance still take the full damage (initial and ongoing)?
Yes, full damage initially and when ongoing takes effect.
I believe even if the PC were somehow immune to Fire damage they would take full damage from the multi-source attack.
DMG pg 7The rules for resistance are not the rules for immunity.
Immunity says you take zero damage of that type. Immunity works fine.
I understand the rules for damage that is "radiant and fire," but how does it work when a creature has resistance of that sort?