so how do we know that crypto exchngers dont sell us the coins what they dont exisit?
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 ?
There is a very informative thread on the matter you want to investigate. Is from a while ago but I haven't seen a better explanation so far.
https://bitcointalk.org/index.php?topic=945881.0Of course it's a matter of trust whether you believe or not to the fact that the exchange you are using will live up to its obligations.
Never forget:
if you don't control the keys, then you have no coins.If I was you I would take some time to study all this.