I'm not sure I understand this fully. Are you saying that if you have an account with 1 million dollars in it, you could actually have 6 million invested with 5% returns, making something like 300k annually minus some margin fees?
Also, you have to be fairly big to get that kind of leverage. $100 million is probably too small but $1B might be enough.
So you put up $1B of your own money, and a bank would let you go long $3B and short $3B. The idea is that your longs hedge your shorts well so you can't lose too much.
Obviously, in the case of Archegos the shorts didn't hedge the longs well.