Generally, using Equipment Viewer components to spawn the prefabs of your equipment on the combatant's game object.
E.g. the 3D RPG Playground tutorials have an example for showing weapon and shield.
Equipment viewers can also change materials on the combatant's game object.
Alternatively, combatants can use Conditional Prefabs to change their entire prefab based on conditions, e.g. switching to a different one based on the combatant's class or equipped armor.