Those radiator end caps can be aluminum or plastic - they are sometimes called side tanks - the core of the radiator, the part that is doing the cooling, is generally aluminum. End caps / tanks can be crimped to the core or welded to the core. Most modern OEM applications will be crimped, cheaper and faster to make - higher performance OEM and high performance aftermarket are generally welded.
Basically a reservoir enables coolant to run/pushed through a central core. Some radiators have one end cap/tank, some have two. My 8th gen Corolla has plastic end caps - they are positioned top and bottom. On your 9th gen Corolla - they are positioned left and right. No real benefit from one to the other, aside from packaging requirements.