Question

Hello Rabbi Michael,
 
The Shulchan Arukh includes enactments and customs that were not written explicitly in the Talmud. For example, some of the laws of the Three Weeks (such as eating meat during the Nine Days) and the customs of counting the Omer. In addition, there are also Jewish laws that were ruled on based on the Zohar.
 
My question is: by what authority did the Rabbi add these? Is his authority formal or substantive? And are these enactments that the entire public accepted upon itself through the religious court, and therefore they have binding force?
This leads more generally to the question of the authority of the Shulchan Arukh in these matters.