In the novels, food production falls into three main areas; Predation on other smaller settlements (both mobile and static), the internal growing of foodstuffs and trading with other towns and cities that aren't worth attacking.
He set off at a run, and soon reached Bloomsbury Park, out in the open
air on the tier’s brim. It had been a proper park once, with trees and
duck-ponds, but because of the recent shortage of prey it had been
given over to food production and its lawns grubbed up to make way for
cabbage-plots and algae-pans.
For ten years it [London] had eaten nothing but tiny farming towns and static settlements in those wet hills. Now, at last, the Lord Mayor had decided that the time was right to take his city back over the land-bridge into the Great Hunting Ground.
Most of the characters would be on the borderline of malnutrition if it wasn't for the prevalence of spinach and easily-grown greens
There was nothing to eat in Grimsby except what the Lost Boys stole,
and the Lost Boys stole only what boys who have no one to nag them
about balanced diets and no-snacks-between-meals eat. Sugary biscuits,
cheap, soapy chocolate, bacon sandwiches oozing grease, thin rounds of
algae bread smeared thick with garish spreads, glasses of ill-chosen
wine that kicked like airship fuel. The only concession to healthy
eating was a tureen of boiled spinach in the centre of the table. “I
always make sure the boys bring back a bit of greenstuff,” explained
Uncle, dishing up. “Helps keep the scurvy at bay.” It spattered on to
Tom’s plate like something dredged from a blocked sump.
There's also some element of trading for food with other (less easily attacked) cities and settlements
“It’s appalling,” agreed Nimmo. “The sort of prisoners we are being
sent these days are just too feeble. If the Guild of Merchants made
more of an effort to solve the food shortage they might be a bit
healthier, or if the Navigators pulled their fingers out and tracked
down some decent prey for once… But I think you have seen enough, Miss
Valentine. Kindly ask Apprentice Pod whatever it is your father wishes
to know, and I shall take you back to the elevators.”
and several "fishing cities" that seems to exist for the purpose of trading food for other materiel with land-based cities and towns.
A rather fishy raft city, whose claim to fame is being the birthplace of the aviatrix Orla Twombley. Ruled by powerful herring magnates, Dun Laoghaire travelled from fishing-bank to fishing-bank all over the North Atlantic, sometimes venturing as far as the Newfoundland Banks, shockingly close to the Dead Continent. It engaged in fierce rivalries with other fishing cities, and some dark rumours claim that it was Dun Laoghaire’s saboteurs rather than pack ice which caused the sinking of Grimsby.
The Traction Codex
In the film there's a machine which recycles effluent back into edible food
I found it very funny that in Scuttlebutt there was a machine in the
kitchen in the background that you don't really see where food was
being made out of poo. And it actually was visually moving and stuff,
and I was just like, 'Who made this?' They went through with it, it's
actually working, and it's not even really featured. And I was just
Mortal Engines Has A Hidden Machine That Makes Food Out Of Poo