I think one of the problems, which was a roadblock for me, is thinking of debt as a physical thing, or somehow seperate from money(cash). The debt isn't anything. There is just an amount of money that the bank owes you and is paying you interest on, just as if you owed the bank money you would have to pay interest on it.
The multiplication of money is really just a multiplication in the amount of money that is owed. It takes a bit of a mind shift but once you get it's actually really simple.
It's like say you have three friends (A,B,C) and you had $100 of cash and they had none.
You lend $90 to A.
A lends $81 to B.
Now you have $10 cash and a promise to pay $90 from A. A has $9 cash and a promise to pay $81 from B. B has $81 cash.
Now say you want to buy something from C for $90. You don't have $90 cash, you only have $10 cash and a promise for $90 from A. But you say to A, "OK, instead of owing me $90 you now owe C $90." And you then take the $90 product from C.
Now you have $10 cash. A has $9 and a promise to pay $81 from B. B has $81 cash. And C has a promise to pay $90 from A.
If you have $10 cash in your pocket and $90 in your bank account, you actually have $10 cash and a promise to pay $90 from the bank.
Hope that's clear.