Downloadable paid content

What I want is that once the player downloads the game which is supposed to be free.

He can buy some levels of the game whuch when bought can be downloaded from the server.

There are some questions:

  1. What is this system called
  2. How can I do this?
  3. Can this be done on a Php server
  4. Can I implement it like when a user buys it, everytime he is going to play a new level the level is downloaded and them loaded

it is called IAP, in app purchase
unity has an IAP service but you could code your own, but is a lot of work Unity - Manual: Codeless IAP
done what exactly? the app purchase or the download levels?
yes you can implement it as you prefer, but my recomendation is that they download the content once they do the purchase not after when they enter the level could be frustrating for the player have to wait