I’ve got only email which also was a login name and my passport information.
From what you have written it seems like you used a centralized exchange to buy your coin; but the issue here now is “are they still operating?” If you don’t remember the name of the exchange you used i think it’s because it was not a popular one and might have closed down their service and that’s one issue you’re more likely to face. And since it’s a centralized exchange there’s nothing you can do to reclaim your coins back if it my speculation turns out to be true.
There is no information about this purchase on my email also.
Does anybody have any idea how to find out what was the site and where the BTC now ?
This is another dead end;
You remember the popular phrase ”not your key, not your coin”? that’s what best explains this situation. It will be impossible to analyze the transaction since it was never made because you purchase it in your cex account and all that was displayed in your account were just number not real bitcoin so unless you moved it to your non-custodial wallet you won’t be able to get anything by analyzing the blockchain chain.
And aside from that; if your come across a transaction with 100+ bitcoins it does not automatically mean that’s your transaction, there is a high probability of people transacting that amount since the value was not substantial back then so this is also another dead end.
This reward is quite enticing with the current price of 1BTC but until you come up with a better lead on this case I’m afraid nothing can be done for you.