How can merkle trees help validate the content of a block in a blockchain

Question

The linked definition states that SVP is a method for verifying if particular transactions are included in a block without downloading the entire block.

If the merkle tree is a method to check if a specific transaction was part of a block, this implies that we do not "trust the block" and are thus checking if the hash of the searched transaction is part of the tree. If we do not trust the block (or the original validator of the block) how come we trust the merkle tree, which could also be faked (or could it not?).

Are we "just" relying on the fact that has values are pseudo random and that it would be hard to create a fake tree with the same root-value, but one different node value?

Thanks!

Answer 1

As long as they are centralized entities then, this is what happens:

Mr X and Mr Y send their crypto to the exchange controlled wallets. The exchanges keep the record in their own DB/ledger who owns what. Then when Mr Y and Mr X make a trade the exchanges record that trade in their own DB/ledger and nothing is recorded on public blockchain. Now Mr X withdraws the crypto from one of the exchanges and it gets recorded on public blockchain (If he withdraws BTC then it will be recorded on BTC blockchain). So Mr X and Mr Y can do thousands of trades without paying network fees as everything happens in the exchange's internal system and they may have per trade fee.