Question: The Authority of the Shulchan Aruch and Regulations

Hello Rabbi Michael
 
The Shulchan Aruch contains regulations and customs that were not explicitly written in the Talmud. For example, the political part of the Mitzrayim (eating meat during the nine days) and the customs of counting the Omer. There are also additional laws that were decreed from the Zohar.
 
My question is, by what authority did the rabbi add this? Is his authority formal or substantive? And are these regulations that the entire public has accepted from the court and therefore have binding force?
This generally leads to the question of the authority of the Shulchan Aruch in these matters.