Financial questions that need an answer

Variable interest rate banks change us is this set by RBA, does this mean RBA lend the money to the banks to lend to us at the variable rate if not shouldn’t the rate be determined by the money market supply and demand.
Then how can RBA influence the variable home loan market when the rates are actually determined by supply and demand forces. I don’t understand this one bit.

RBA should have no effect on the money market right cos the interest rate should be determined by the money market supply and demand.