How does the technology for displaying ads over public WiFi work?

Question

contextual advertising

Good day to all.
At one time I moved to Europe and now I want to do my own thing. The essence is the creation of pop-up advertising through public WiFi networks (restaurants, cafes, access points in the city, fitness rooms, etc.)
I would like to talk with a specialist who knows how it works, how it is configured and implemented? I know that ubnt has UniFi, but I don’t fully understand the principle yet. Thanks

Answer 1

Code injection in html. Doesn't work with HTTP, won't work with HTTP2. It is configured by intercepting packets coming from 80 ports and adding your code before the /body tag.

Answer 2

A hotspot solution is implied, i.e. when connecting to a point, the user opens a browser and an advertising page opens, or is the option considered when advertising banners pop up at some intervals during surfing in the browser?