Polibi RS-TSCSCBB-BN, RS-TSCSCBB-W User guide

Hello! I've reviewed the assembly instructions for the Bunk Bed WF290140. This document includes important details on how to assemble the bed, safety warnings, and a complete parts list. I am ready to answer your questions about the assembly process and the features of the product.
What is the recommended size for the upper bunk mattress?

What is the recommended size for the lower bunk mattress?

Where can I obtain replacement parts?

What should I do before starting assembly?