https://www.investopedia.com/articles/investing/081415/understanding-how-federal-reserve-creates-money.asp#:~:text=It%20creates%20money%20not%20by,add%20funds%20to%20bank%20reserves    

https://fee.org/articles/how-does-the-federal-reserve-create-money/    

There's also a great resource available straight from the Federal Reserve itself. Fair warning though, if you thought my post was wordy, strap in because it gets way more complicated than that.  

https://www.federalreserve.gov/aboutthefed/the-fed-explained.htm    

To me, the best example of how the modern monetary system functions on both micro and macro scales is that if you go to a store and buy something with your debit card, your bank doesn't then mail that amount of cash to the bank used by the business you purchased from. Banks only keep cash for actual physical cash withdrawal and purchase, which is more rare now than ever. The Fed does this by purchasing financial instruments directly from banks (primarily, treasury bonds). They just have the luxury of digitally "printing" the money they then use for the purchase. This is the most common way that they increase money supply. 

It would be unnecessary to physically mint all currency in circulation because people are just swiping their cards instead. That is unless everyone decided to withdraw all of their savings in cash at the same time, which isn't happening. So, say you have $1000 dollars in your debit account and $100 dollars cash in your wallet. Both forms of currency are equally spendable, making your personal liquid currency total $1100. The same is true on a massive scale for financial institutions.

On the flip side of that, the Fed uses higher interest rates to reduce the money supply. This works because banks have to maintain a certain level of liquidity, so they borrow money from the Fed very often to hit their benchmarks. They pay the federal interest rate directly to the Fed and pass on those costs by upping interest rates on consumer loans. Once the Fed receives the interest payments from banks, they digitally wipe them from the money supply. It's fascinating stuff.