Actually sp.io and whale look very different. The site I compared it with is 1:1 the exact same sports book, and I mean 1:1. I was surprised when I saw that actually so maybe there is some sort of package or whatever casinos can buy, so I was just wondering because I have never seen the same book twice.
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There are indeed providers for such sportsbooks, in the case of whale.io it is BetsyGames (
https://betsy.games/). Companies like BetsyGames specialize in order books, overviews, leagues, ... and then sell the views that customers (casinos, sports betting providers, ...) simply need to integrate into their sites.
For the casinos, of course, this has the huge advantage that they need fewer developers (and know-how) themselves. The order books not only have to work well, but also (and above all) have to be well secured in terms of security so that players cannot take advantage of exploits etc.