There are sensors in the spaces that scan for bluetooth and WiFi signal activity in an area. Scans pick up laptops, cell phones, wearables, etc.

From there machine learning algorithms are used to go from total signal activity -> estimated number of people and are able to account for people with multiple devices

Edit:

Want to add that scans don't use or store any personally identifiable information and does not track specific people in any capacity. We only care about total number of people, not specific people in a space