It could be one of two things:

One thing that they do is they roll out marzipan (almond paste) like a pie dough and put it over the cake itself.

If you were thinking of making a cake that is smooth, marzipan is kinda expensive and to me at least it tastes funky.

Another “smooth” thing that they do on wedding cakes is after you frost the cake, there are little lines everwhere, right? So you dip your spatula or whatever in water and smooth out the wrinkles.
The frosting can be any type of frosting, like the Betty Crocker ones in any grocery store

Okie! I hope that I helped because it was the best I could do!
Good luck on your cake!

=o) Tennis girlie