All Mossberg shotguns made at least since the model 500 was introduced have aluminum receivers. They haven't made a steel receiver shotgun for many years.
The Maverick is essentially Mossberg's "super budget" gun, designed to sell at an absolute rock bottom price.
Major differences between it and the Model 500 are, it has the safety located in the trigger guard instead of on the receiver like the Model 500, and the action bar assembly is made as an integral part of the forearm.
As I recall, the Maverick is assembled in Eagle Pass Texas from parts made in Mexico.