The main thing that the management of the casino will do immediately is they will first carefully investigate their users to see if they have not violated any policies and rules in their gambling.
That's the first thing that they'll do before they release the money to the gambler and that will come into a long investigation if they suspect that the user won that by surprise. They're always suspecting their winners if that's big money to be withdrawn.
The battle will be long into the courts if the winner don't have a lot of money to pay for all of the cost of those trials and the casino might have just to settle it for a lower amount but it's up to the petitioner/gambler if he'll agree on that deal.
But one thing we have to know is that every gambler is to win big so casino companies, management and customer care service agents should also know that so that when a gambler wins big, it should be a surprise to them, Because that the normal norm for all gamblers. Now the question says,
Assuming you gamble on a mid to high-rated gambling website and you won big but they refuse to pay up, what and what are the legal way to make sure that the platform pays up your money?
The legal ways as it is said before, the first thing is that you the gambler did not violated the rules and the company refused to pay you because you win big, sue them in the court of law and present all the necessary documents and stake analysis, The date of the stake, amount used in betting and amount won. The site domain and the statements made by the company base on the refusal of payment. Everything communication between you and the company must be presented to your lawyer