To add one thing, well two, casinos cannot edit or adjust the RTP of a game at all. Only the provider can. Most providers only offer an RTP around 96%. Some (novo) avg at 95%. Some offer variants (Red Tiger has 89/93/96 i believe). The casinos pick the ones they want and thats what you play.
As for “billions of spins” – it may feel like that but i play almost every day, real money, either testing or playing for me, and after tracking i guessed i had done about 1mil spins. Turned out was 160k. Feel like more!