so how do we know that crypto exchngers dont sell us the coins what they dont exisit?
I think that the best indicator is that the exchange is old/established and nobody complained they cannot withdraw their coins from the exchange (to their own wallets).
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 ?
You cannot really know who is trading what amounts. There are plenty of exchanges and an entity can spread the orders between them.
If you buy/sell it doesn't matter if you use a reputable exchange, see your first question. Most exchanges match the funds between buyer and seller, so if they didn't deposit they cannot trade.
A much better question is who is
holding how much. And there some are probably lying (like the story that USDT is properly backed) and some are probably telling the truth (Square), but only 3rd party proper audit can tell the truth.