Which ones are reliable?
Which ones are people actually using?
How do they work?
Exch and others (but I can't vouch for them since I haven't used them before) but it seems they have been used by others and they didn't find any fault.
Exch is the one that I can vouch for as one of the best out there and you shouldn't be scared using them since you won't encounter any scam provided you're accessing the exchange from the write URL. How instant exchanges work is right there in their name, they're instant so they exchange your coins for you instantly. Having more knowledge how they exchange the coins for you instantly shouldn't be your problem since they're solving your problem without any issues but just know any coin that they have listed on their platform means they have them available to send you based on the exchange rate instantly. When using instant exchange, you're transacting with the exchanges and not another trader.
Some respond recommended with exch as instant exchange not required with KYC, seems its good option for OP looking for instant exchange and not regulation of verifying account make its easily for withdrawing or deposit fund there.
Not requiring kyc makes it more convenient to use instant exchange if we value our privacy, let us not make it a norm to use centralized exchanges that needs our kyc that they can sell to the biggest bidder behind our backs. The government can always take CEX to court and if they win, they're obligated to provide the government with any information that they need on a client of theirs.