3. They are totally random, but they are based on a set of rules that ensure the Casino always wins overall. Best example I can give is the following.
I have a dice. It is random.
You pay £1 to play
If you roll a 6, I will give you £5, anything else and I keep your money.
The result is random, but statistically, the RTP is 80%, because 6 rolls cost you £6 but statistically will only return £5.
So it's the rules that introduce the concept of RTP on to a truly random result.
4. No. TRTP isn't a 'thing', it's just a result of the mathematical model of the slot (see the video).
5. We used to use Flash for the front end and Java for the back end. When Flash died, it was HTML5 for the front end (which is still true) and Java for the back end. I imagine Java is still a popular choice, but basically any language that can build a web service would do.
Going back to question 3, I have watched the video and understand what you said, but it also might depend on the provider, right?
I say that because I have seen a video of a slot developer, however, it was for physical slots in the casinos where the slot had a "cashier" per bet size and will balance the "RTP" accordingly.
Another thing I'm aware of and that's why I said it might not be totally random is that some providers track the number of spins, for example, Pragmatic Play has a counter for each spin you do:
Regarding question 5, the software, the backend isn't an issue for me, I was wondering about the most common engines for the frontend, once when browsing the Relax-Gaming careers I saw two engines, pixi.js and Phaser, was wondering if what is the most common in the current days for slot developing.
But since you used to use flash, I think you might have the industry few years ago already.