RTP is just the average result, that's it. There's no scam or magic involved.
All slot spin and original bet results have a fixed probability. All slot results are pre-programmed in. The casino provider has a full table of the possible results, and the average of that is the RTP. The whole "billions of spins" thing is just marketing, but it's not wrong.
As for how bad you're running, that's just bias. When you hit a bonus immediately that pays 300X, you're running at like 1000% RTP. Nobody says anything about that. People have this delusion that EVERY single gambling session has run at RTP or BETTER, which is insane.
I've seen people go on huge heaters where they 10X their balance. Then they start spinning a slot and run at 50% RTP over 500 spins and complain about it. Pure delusion.