In the default header setup Divi places the social icons in the secondary header bar (the thin strip at the top with the phone / email icons). However, you need to first enable this at:

Divi > Theme Customizer > Header & Navigation > Header Elements > Show Social Icons

This will include Divi's built-in social icons, as well as any icons configured in Divi Booster at either of:

Divi > Theme Customizer > Divi Booster > Social Media Icons
Divi > Divi Booster > Site-wide Settings > Icons > Add more social media icons