I can offer a little insight here, being on the other side of the fence.
When it concerns network jackpots (these are jackpots shared across the entire casino industry, Meha Moolah being the most famous example), it is indeed the game provider that will invoice the casinos (also often called operators in this context) with jackpot contributions throughout time, and it is the game provider that collects these funds and "holds" them, until the jackpot win triggers. When it triggers, it's the game provider that sends the funds to the casino, who then pays the player.
In all other instances (including local jackpot hits), it is the casino that pays the player. In a typical contract, the game provider has no financial liability whatsoever, not towards the player and not towards the casino. Should a slot pay-out a million in a given month, it will only mean 0 gross income for the game supplier in that given month, not an expense to cover that win.
On the contrary, even if the slots pay-out a million on Casino A in a given month, the game provider can still invoice the revenue that is generated on Casino B - Z in that month, without it being lowered by the win on Casino A.
So to answer the question: if somebody wins 50k from an online slot, it is the casino that will pay those funds to the player, not the game provider, except when it concerns a network jackpot (but these amounts are typically much higher than 50k and have already been paid for historically). The financial liability towards the player is always with the casino, which is why it is also this entity that holds the gaming license for example.
