How are retail forex brokers different from betting shops?

Both lose when their clients win.

Not true at all.

I used a retail forex broker for several years to hedge currency risk that I had from cryptocurrency trading on multiple fiat markets. What I was doing was effectively arbitrage. If I had a loss on a retail forex trade, it would only be because I had a larger gain on a cryptocurrency trade elsewhere.

I've used retail forex to hedge currency risks in various things I've been doing for decades. I do not see any reason to think my broker was losing while I was winning.

Forex is not a zero sum game. Forex reduces risk in foreign currency transactions, facilitating global trade.

I'm about about simply buying and selling currencies using online brokers. I'm not asking about futures or spread betting.

Note that you don't refer to a company that buys and sells currencies as a "broker".

They're not brokering anything. They just buy and sell things - exactly like a used car yard.

Both lose when their clients win.

That's wrong. The only time currency exchanges lose is in the (incredibly rare) case where the market changes dramatically, very quickly, and they are left holding stock.

(Exactly like a used car yard. If there's suddenly no demand for a vehicle which was popular last week, you might get stuck holding a couple of them.)

Both win when their clients lose.

Ditto. Currency exchanges don't even know if someone they sold some currency to "won or lost".

Say you buy, and later sell, some CHF to an exchange. (For you that may have been part of a "trade" to make money.) They don't even know or care that you did two transactions. (Indeed, it could be you did the two transactions with two different exchanges.) They make money separately on each of the two transactions.

Both set prices on two sides (team A and team B for sportsbooks, and bid/ask for forex brokers) at which their clients can transact.

That is not really correct, currency exchanges don't really "just pick" a price. They look at the "market" price of (say) USD/EUR (the "market" being the massive forex market) and then use that price with (as you say) just adding a percentage.

(Thus: observe that for the most currencies, that percentage is lower {because there's less risk of a fast move} and for obscure currencies that percentage is higher. Exactly like, if you sell Toyota Corollas the markup is lower (near the bone) than if you sell some obscure model.)

It depends on the country you are in. In the United States, you are quite correct. They often carry a commission too. And, like a regular bookie, they "lay off bets." They also offer leverage.

In other countries, where they are heavily regulated in terms of things such as price quotations, they are more like commodity broker-dealers. Of course, a commodity broker-dealer isn't that far from a bookie on the dealer side of the fence. The U.K. market is reasonably well regulated from that perspective.

If, for some reason, I wanted to trade in the retail FOREX market, and I did not want to use a proxy such as buying Euro-denominated bonds, currency futures, or options, then I would fly to the United Kingdom and open an account.

If I were not carrying balances large enough to justify a plane ticket, the research time, and stay in London in order to open an account, then I would not be in the FOREX market.