In addition they have business accounts which don’t have any of the costly features like interest, free cash deposits, etc so they’re profitable for Starling as they profit off the interest (especially relevant for business accounts as they might hold high balances keeping money aside for taxes and VAT) without giving out many costly features like cash deposits (the only overhead there is support and foreign ATM withdrawals).
Domestic ATM withdrawals are a cost for Starling as they have no network of their own; they're not even part of Link.
I'm not sure that the paltry interest they can gain from business deposits will be enough to make those accounts profitable (they just announced another £60m of funding this week).
I'm a relatively happy personal customer of Starling, so I'm not against them, but I do think it's a bit optimistic to say Starling's business accounts are profitable based on no evidence.
I was a huge Monzo fan, converted dozens of friends to it and was one of the first ones to get their beta current account (which I immediately started using as my main and only account despite it being in beta).
Nowadays however they seem to have trouble making profit, but instead of focusing on their existing customers they pour money into more marketing even though they can’t actually support all of them and now customer service response times are measured in days. They feel like a bullshit social media company with no real (aka profitable where people are happy to pay for it) product and are just trying to inflate their customer numbers no matter what.
Now I bank with Starling. They aren’t as “nice” as Monzo used to be but are definitely a lot better than the current Monzo both in terms of support and UI (Monzo redesigned their UI and it’s a shit-show now). They also offer business and Euro accounts if needed.