Smart contracts are complemented under certain conditions.

Lets see some transactions by OpenSea

This is their smart contract code

Now. listen properly, assume that 2 of you are playing a game and you code in a way that whoever wins will get the money. After the games ends and 1 person win, the winner will get the price due to the code you wrote. There is no Middle man and this is what smart contract does.

Web2.0 vs Web3.0

Now, all the big companies have their own data servers but in Blockchain which uses Web3.0, there is just 1 data server . Thus things become decentralized .