TLDR: don't trust anyone offering you a fixed rate of return, if it sounds too good to be true then it probably is. Great advice but very much over-complicated here.
A fixed return is possible if the creditor is absorving the risk.
Creditor runs risky investments using your money, but gives you fixed returns in exchange for lower yield. This is fine as long as there's oversight over liquidity to guarantee the fund manager didn't lose it all - it'd then become a ponzi.
Possible yes, just not the rates that people offer in many circumstances. This post hints at it. If someone could even make only 1% a day, that compounds to be a 3700% increase in just a year and a 142700% increase in 2 years. Why if anyone had a system that could create so much would they offer to let someone else access it for just a small fee or anything else? Anyone offering a guaranteed return of over about 20% (even that is precarious) per year is almost certainly working some sort of an angle.