dex ? no never used
sorry for the late answer, dex is an exchange that allows you to trade your assets without doing the kyc. it works the same as a market where the seller and the buyer meet directly, that is why they are required you to connect your wallet because your wallet will be used as a base for your transaction. or you can try to read this article
https://coincenter.org/entry/what-is-a-decentralized-exchange to know about it in detail.
SAI is a single collateral DAI coin
DAI is a multi collateral DAI coin
Both are different.
Yes, they are different as I've looked into CMC. I don't know the history of it but there's this message that appears on SAI and DAi on CMC. And I've found this blog:
Single-Collateral Dai to Multi-Collateral Dai Upgrade Timeline and ActionsAnyway, I still have one question for op that he didn't answered which is both are stable coins. What does DAI/SAI offers if USDT is already a stable coin?
both of them are the same, they are part of maker dao.