Who decides where, what and how many commercials are in shows and movies?

The advertisers decide when they want their commercials air, and during what shows.

The station. Say you have a fox affiliate showing a football game, they have their local commercials and national commercials.  They have a constant live feed of the game and fade in and out when needed.  Commercial blocks are put together before the game based on what type of ads, length. It's sorta of like a puzzle as they target 2 to 3 minutes per break. Some ads are ran at the end of a quarter and those cost more.  On a prerecorded show like the price is right, they have a target time of 42 minutes and a set number of breaks and the producer signals the host but after 1000 episodes, they just know