CEX (centralised platforms) are official crypto exchanges that operate in the market and comply with all the rules and requirements in this field.
Pairs such as [https://whitebit.com/trade/BTC-EUR BTC EUR], BTC USD and others are available only in CEX, for transactions with fiat currencies require users' verification. KYC (know your client) is a procedure of client verification and checking that takes place on centralised platforms to ensure that a client is not associated with illegal activities or fraud schemes and does not pose any danger to other exchange clients.
Decentralised crypto platforms do not require KYC, while at the same time, they do not promise safety for clients' funds. That is the main difference - on a centralised exchange, you can convert BTC to EURO and withdraw funds directly to your bank card, while DEX does not offer such an option.