So okay, this story didn't happen to me. It happened to a friend (let's call him Bob) of a friend (I will call him Sam).
But he has an app (I don't know what app, because I heard it from Bob and he doesn't know either) which shows that Sam has doubled his investment (9000 EUR), but he cannot withdraw it now and these people (from Russia) are calling and rushing him to invest even more.
Bob asked me about that, I said that this is some nonsense. There shouldn't be people calling you and asking to transfer money. All is too fishy.
This is surely a scam and it would be best to stay away from it! Most of these applications only make you millions on their platform without being unable to withdraw the funds earned. This sort of schemes is actually archaic and only crypto noobs would fall for this. Stay away from quick gains guys, that's one of the quickest ways of falling vulnerable to scammers.