An earlier MacRumors article dated from 2015 claimed that the visitor center building would be at 10700 N Tantau Ave, one building north of the one you mention.The visitor center building (store and caffe) must be located at 10600 N Tantau Ave.
I remember reading about a viewing area on the roof. It won't be in the main circle building.
Either way, it appears that the visitor center and cafe will be sequestered from the main ring building and campus by being on the east side of Tantau Avenue. That probably helps with security, liability, permitting, etc. The underground parking garage for the visitors center will likely have no access to the employee garage.
Unless you are a qualified guest of an Apple employee, vendor, business partner, etc., the general public will have no access to the ring building.
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