It depends on situation. As i know in some casino is used free code - it is possible to load on github.
No, it doesn't depend on the situation. You're pulling info out of your ass and are just guessing, and then half of the forum will quote you and believe you.
The same reason you can't find a popular slot game code on github is the reason you can't find a Zelda game on github = trademarks, copyrights and, of course, money.
Pragmatic, Netent and others don't want to share their templates because then copycats will appear. It already happened with Book Of games once.
And no, it's not because of RTP mechanics, all of the certified games are transparent in that and to everyone actually reading it can understand that they are here for entertainment only, not for making money.
And how do you imagine auditors, people in their mid 20ies knowing how to read code?
No man. You're dealing with corporate processes, trademarks, certificates etc. Evolution Gaming Group (who owns most of the popular slots) is a *publicly trading company* = you really think their business model is scamming people? It's as every other corporate business out there, it has a formula that entertains the user and makes money, that formula is published openly. The code though? No.
Source: I work in the industry
If a user on stake.com lose 1000 bets on the sweet bonanza slot, can a user from bc.game win big on that same slot?
That can happen (one operator having less luck with a game than the other operator), but it's not correlated. Separate casinos, separate accounts, separate pool.