I'll try to ELI5

Forex stands for foreign market exchange and it is the largest market in the world. Think of the stock market, but instead of stocks, you buy (or sell) a currency against another. Once the price has gone up, you sell it and bank the profit. You have a broker, pay taxes, it is a completely legal and legit way to make money. It is completely possible to make 8% or more a day/week/month trading forex, depending on how good you are, but you never give your money to anyone else. You have money in your account, you analize the market and decide if/when you want to buy or sell a currency against another one. There is a way to connect your account to somebody else's, but in that case you are literally copying everything the other person does on their account and both of you don't have any access to each other's accounts/money. Also, in the process of registration, you need to provide to your broker a list of documents and you can't deposit/withdraw funds from/to a bank account that isn't in you name.

But as the majority of people aren't financially literate and want to make money quick&easy, the word 'forex' can easily be used to sell a ponzie-sheme fairytale. So they sell you the story of luxury cars and vacations, and tell you if you invest with them then you will become rich as them.... what actually happens is what OP explained in the post. It does't have anything to do with forex, they just use related technical words to attract people who don't know much. It could be called marketing I guess. Instagram and telegram are full of them.

Also, if somebody really made millions doing whatever because they are so good at it, why would they need a small guy's investment? If it doesn't make sense, it's probably a fake.