The "icing" and decorations are usually made with Rich's Bettercreme (also called Pastry Pride). This stuff is sold in cartons in the frozen food section--thaw it and whip according to the directions on the carton.
The store cakes are layers of sponge cake or cake mix cake. Once they are cold, line the same pan with plastic wrap and build the layers back into it. You put in cake and add the softened up ice cream in layers and any other stuff like chips etc. Then you chuck the whole thing back into the freezer after each layer of ice cream.
You need to get your covering stuff ready (whip it, have pastry bag and tip) before you take the cake out of the pan. You ice quickly and freeze that layer. Then you decorate as fast as you can but make a neat job. Then the whole thing waits in the freezer for the party.
The store cakes have some air brushing but you gotta be good to be quick. For home, you would be better off having a sugar shet printed with a fancy image. Put that on JUST before you serve the cake or it will bleed all over.