Is it permissible to work in the field of advertisement design by searching for free, ready-made templates, imitating their layout, and then completing the design and selling the final product? What is the ruling on that?

There is no harm in imitating the layouts of free templates and designs, with the designer adding their own touches to make the result different from the original. This is because it is not considered an infringement on anyone's rights; rather, it is a utilization.