Those gigantonormous progressive jackpots on machines like Megabucks, Quartermania, the Wheel of Fortune, Jeopardy, The Price is Right, etc., are paid by the manufacturer of the slot, in this case, IGT. If ever someone’s stars do truly align, IGT (the game's vendor) would send a representative to authenticate the win, and then pay off the winner. To date, IGT MegaJackpots have created hundreds of millionaires, and they have awarded over $3 billion in major jackpots.
Casinos also have their own proprietary progressive machines, typically with their name and logo on the facing. Although the casino sets the percentage they want returned to them when placing an order with a slot manufacturer, the casino would be responsible for the payout.
By the way, Susie, to grow the progressive, a portion of each bet made funds the winning jackpot.