How are on-demand ads implemented?

Question

Internet advertising

Greetings!
In some applications or sites, it is possible to get a paid service for free by watching an advertising video or banner for a specified time, for example 30 seconds. How to implement this at home, which advertising network provides such an opportunity?
Example: I have a website with products posted by users themselves. There is an opportunity to "apat" for 3 rubles a day. I want to make it so that the user can not only pay with money (implemented), but also offer an alternative: click the "view ad" button. The user clicks, watches the ad, and then receives the service. Well, the site receives a small amount of money for viewing ads. Something like that. Exactly this approach was seen in the application CCleaner and Duolingo
PS The most interesting is not even the technical implementation, but what advertising networks provide developers with such an opportunity?

Answer 1

The format is called rewarded video, look for ad networks with a similar format.
AdColony does this.

Answer 2

Programming is simple:
- a player for viewing, which makes you wait for a specified time, reporting to the server about the facts of the beginning and end of viewing
- the player can be universal, showing arbitrary content
- on the backend there is an entity with access permissions. The model adds a temporal permission based on a signal from the player's view model that a view has
taken place - bind view sessions to IP, cookies and/or authorizations
Total four models:
- player content
- views
- permissions
- target content
Views and permissions can be combined.
"Something ready" saw on getcourse.