How can I create the wall shelves where the fireplace is?

You could use the customized modelling section of the software. For more information or tips on how to use it, check out some videos provided by the amazing Homestyler staff.

Hi, the shelf was made with "customized/interior modeling" tool. You could check out the "Basics of Customized Modeling" tutorial in our "Learn" Page: https://www.homestyler.com/learn/video