One of the most eye-catching things in Stranger Things 3 (surprisingly not Billy) is actually the Starcourt Mall that is featured prominently throughout the entire season. Both above and below ground where the Russians have a secret underground base (yes you read that right).
Normally something like the Starcourt Mall would be created on set, but the team working on the show actually used a real mall. They found a semi-abandoned mall on the outskirts of Atlanta, and esentially gutted it and renovated it inside and out. Every part of the mall you see is real. Operating out of the abandoned wing of the mall, if you were to head to the other wing of the mall there would be real 2019 people shopping at modern stores.
Considering how secretive the Stranger Things team has been in the past, it is truly amazing that filming directly beside everyday shoppers didn't result in any leaks.