Assume that a creature is proficient with its armor, weapons, and tools. If you swap them out, you decide whether the creature is proficient with its new equipment.
For example, a hill giant typically wears hide armor and wields a greatclub. You could equip a hill giant with chain mail and a greataxe instead, and assume the giant is proficient with both, one or the other, or neither.
See the Player’s Handbook for rules on using armor or weapons without proficiency.