ESPN has an app you can download to a mobile device. Smart TVs have it as does AppleTV and maybe Roku or Chromecast. Not sure if FOX or CBS do this.

NBC runs a mutimedia access website during its games from their website. http://www.nbcsports.com/snf-all-acc...night-football
That is where the alternative camera views live for their games.

If its an app, you usually need to activate access by choosing your cable provider and entering in an access code to activate the device.