Here is the short version:
Epstein offered the post puberty teenyboppers he seduced and/or bribed to the rich people he knew. He invited lots of interesting people - artists, scientists, politicians, rich businessman - to his exclusive parties. There were always these young girls around. There was always a free bedroom. There were also cameras in place. When one of the rich guys messed with a girl Epstein would blackmail him.
But instead of taking cash he asked them for investments in his offshore hedge fund. For someone who owns billions it is peanuts to put a few dozen millions into a fund. It is legal. The money isn't gone. It will even bear interests.
Epstein is not known for having done much currency trades or other larger Wall Street transactions. His company is small, he didn't work a lot. It is likely he mostly re-invested the money in a simple index stock fund which follows the S&P 500. Those type of funds brought over the years quite a good profit.