also , if an exchanger ask for kyc , you can use temporary email to hide your real identity.
That's not what KYC is. KYC is demanding pictures of driver's licenses, passports, bills, tax forms, paychecks, selfies, and so forth. You can't bypass it with a fake email address.
dex or decentralized exchange were already anonymous , so why need for tor or vpn ?
Just because they do not collect your personal details or ask for KYC, doesn't automatically make them anonymous. They will of course collect your IP logs, browser fingerprints, and other potentially identifying information. If you are trading fiat, then depending on your chosen method you may also reveal your details to the person you are trading with (using a bank transfer, for example). If you want to be anonymous in your usage, then you should be using Tor and an anonymous payment method.