so how do we know that crypto exchngers dont sell us the coins what they dont exisit?
By avoiding trading shitcoins in shit exchanges. When you buy bitcoin from an exchange like Binance or Kraken, then there is a good level of confidence that you actually bought real bitcoins. The next step would be to withdraw them to an address whose private keys or backup seed phrases you have access to. If those Bitcoin's haven't hit your bitcoin address, they are technically not yours.
and how do we know those big funds who bought or pretinding to buy or sell some big ammounts of cryptocurrency `?
how do we know they dont lie us ?
The Good thing about bitcoin is that everything concerning transactions is viewable through bitcoin blockchain explorers. If a company claims that they bought and hold 10,000 BTC, then they should be able to prove that they own such an amount by providing the address that hold the bitcoins and sign a message for you to confirm by use of a blockchain explorer.