Yes you can visit. There is also a cafe named Caffé Macs. There you can drink coffee.
No, apple park is closed to to all tourists all the time. It's closed to the public. You can visit the visitors center across the street witch is basically just a apple store with a coffee bar inside.
You can visit the visitor center anytime when it’s open. But if you want to visit the office, you need to arrange a tour from your Apple employee friend.
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