Which ones are reliable?
Which ones are people actually using?
How do they work?
Instant exchanges, I don't understand what you asked.
Exchanges have two types: centralized, decentralized. Centralized exchanges are more favorite by people because these exchanges have higher trading volumes than decentralized exchanges. They have more trading pairs and there is less risk of rug pull with projects listed on centralized exchanges.
With decentralized exchanges, trading volume is usually lower and risk of rug pull is bigger. Trading on decentralized exchanges require available Liquidity pool and it is risk of rug pull if the project team rug the liquidity.
Exchange can take some seconds to finish on decentralized exchanges because your trade is an on chain transaction, it must be confirmed to complete.
On centralized exchanges, your trade is off chain, not on chain but your order can be filled or not filled, depends on order type you use.