There are two banks in every ACH transaction, the originating bank (ODFI - Originating Depository Financial Institution) & the receiving bank (RDFI - Receiving Depository Financial Institution). They can be the same (I’ll get to that below) but frequently aren’t. Depending upon how an ACH Originator is set up with their bank (the ODFI) they can initiate credits (ie. direct deposit, bill payments for individuals, etc) & possibly debits (car, mortgage, utility payments, etc.). The ODFI isn’t going to let just anyone initiate debits & will do appropriate due diligence when approving a customer to initiate debits. This can include looking at financials & the type & size of business. The bank internally assigns a business a risk rating including limits on how much the business can initiate & how much, if any, of that must be prefunded. There are limits per file, per day, & in total. (ACH traditionally was a multi-day transaction from initiation until effective date; there is now same-day ACH). Obviously, the electric utility or cable company will have a much higher limit than a small professional services firm that bills you; an accountant, lawyer, engineering office, etc.
The ODFI is bound to the ACH rules & anyone who initiates ACH transactions (txns) is bound to the ACH rules vis-à-vis the contract they sign with the ODFI to participate in that service. One of the items in that contract is that they will only initiate duly authorized txns & for the approved amount. In the older days of ACH that was an actual wet ink signature on a dead tree piece of paper from you, the party that was being debited. Obviously, electronic (or telephonic) approvals are allowed now.
At the appropriate time(s) of day, the ODFI sends A file to the ACH operator who slices & dices it & creates files to be sent to the various RDFIs, much the same way that one drops a bunch of letters/packages off at Fed-Ex/UPS/USPS for delivery to various addresses in various states.
The RDFI then posts the various debits & credits to their customer's accounts. You look in your account & see your paycheck/direct deposit, as well as various debits, one for your mortgage, one for your car payment & one you don't recognize at all. If this is a consumer account, you have 60 days to notify the bank that there was an unauthorized txn thanks to [Reg E](https://www.google.com/search?q=Reg+E&oq=Reg+E&aqs=chrome..69i57.631j0j1). You notify your bank, they send a (code R10) rejection notice back to the ODFI who then reaches out to their originator & basically says, "Show me what you got". If the originator can't produce appropriate authorization, that txn gets reversed & you get your money back. The ODFI then debits the Originator for the amount of the txn. If for some reason, there’s no money left in the Originator’s account, the ODFI bank eats the loss; banks don’t like to eat losses! There’s basically two reasons this can happen, bankruptcy of the Originator or fraud. In the latter case, it’s a federal offense & you have lots of resources & G-men coming after you. Yes, unauthorized txns happen in the wild occasionally; especially when an acct # typo results in an incorrect but valid acct # but if an originator comes anywhere close to the limits the ODFI stipulates in the contract, they’re going to be monitored closely & possibly have their acct closed. The banks have a compact where they report closed accts & the reason; good luck opening a new acct at a different bank.
Further, if an ODFI has too many returns for that reason (<½%), across ALL of their originators they can lose the ability to initiate ACH txns, which means they’ll most likely lose all of their business accounts. If NACHA (the ACH governing body) were to do that, you’d better believe that they whole host of different bank regulators would be scheduling audits & going over the ODFI’s policies, procedures, & practices with a fine-toothed comb. No banker wants that.
The above being said, a variable rate bill much higher than expected, like the TX electric co bills last month, or back in the olden days (2019) you’d occassionally hear a story of someone who went to a different coutry & racked up a multi-hundred/thousand dollar cell phone bill because of roaming charges & not being on a local plan in whatever country the trip occurred in is NOT unauthorized. They take their billed amount & you’re now screwed because you now can’t pay your rent/mortgage. Nope, I don’t let any variable bill (utilities) to make a debit to my account other than possibly a one-time debit (where I get to approve the $ amt being debited)