Welcome to the Cake Decorators Q&A
How to stack a buttercream cake
I watched how to stack the cakes. It looked like they were fondant cakes. How would you stack a buttercream cake if you're putting your hands on each of the layers? That would leave imprints on the buttercream.
Refrigerate the cakes first to firm the buttercream up then stack in the usual way. If a few marks do appear after handling, use a warm palette knife or side scraper to gently smooth out the buttercream. Some people use a piece patternless kitchen towel or a napkin against the cake to hold it steady. The warm knife method works a treat.
The are lots of youtube videos too which may help. To see them search 'how to stack buttercream covered cakes'.
Hope this helps.