How can I create a menu link that shows only to authenticated users?
Long story short, you create 2 different Menu versions and give each version different visibility rules. 


All you gotta do is configure your menu link block



Then in Roles, check mark ☑ Authenticated users and  you may want to also ☑ Administrator



Now create another Menu (/admin/structure/menu) which does not have the "add node" link. Then go to blocks (/admin/structure/block) in Primary Menu (Assuming you're using Bartik, may have a different name if you're using a custom theme) click add, and add the Menu you just created and configure it. 

This time for Roles, only check mark ☑ Anonymous


  PS: In pages you can restrict so it only shows up in certain pages.